Silicon nanowire field-effect transistor-based biosensors for biomedical diagnosis and cellular recording investigation
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Nano Today (2011) 6, 131—154
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanotoday
REVIEW
Silicon nanowire field-effect transistor-based
biosensors for biomedical diagnosis and cellular
recording investigation
Kuan-I Chen a,b,1, Bor-Ran Li a,b,1, Yit-Tsong Chen a,b,∗
a
Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan
b
Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei 106, Taiwan
Received 31 October 2010; received in revised form 18 December 2010; accepted 7 February 2011
Available online 8 March 2011
KEYWORDS Summary Silicon nanowire field-effect transistors (SiNW-FETs) have recently drawn tremen-
Silicon nanowire; dous attention as a promising tool in biosensor design because of their ultrasensitivity,
Field-effect selectivity, and label-free and real-time detection capabilities. Here, we review the recently
transistor; published literature that describes the device fabrication and biomedical applications of SiNW-
Protein—protein FET sensors. For practical uses, SiNW-FETs can be delicately designed to be a reusable device via
interaction; a reversible surface functionalization method. In the fields of biological research, SiNW-FETs are
DNA hybridization; employed in the detections of proteins, DNA sequences, small molecules, cancer biomarkers,
Peptide—small and viruses. The methods by which the SiNW-FET devices were integrated with these repre-
molecule interaction; sentative examples and advanced to virus infection diagnosis or early cancer detection will
Biomarker detection; be discussed. In addition, the utilization of SiNW-FETs in recording the physiological responses
Three-dimensional from cells or tissues will also be reviewed. Finally, the novel design of a three dimensional (3D)
localized bioprobe nano-FET probe with kinked SiNWs for recording intracellular signals will be highlighted in this
review.
© 2011 Elsevier Ltd. All rights reserved.
Abbreviations: AFM, atomic force microscopy; ATP, adenosine triphosphate; CA, carbohydrate antigen; CaM, calmodulin; CEA, carci-
noembryonic antigen; CgA, chromogranin A; CNT, carbon nanotube; CVD, chemical vapor deposition; DNA, deoxyribonucleic acid; EDTA,
ethylenediaminetetraacetic acid; FET, field-effect transistor; GSH, glutathione; GST, glutathione S-transferase; His-tag, histidine-tag;
miRNA, microRNA; MPC, microfluidic purification chip; NTA, nitrilotriacetic acid; PBS, phosphate buffered saline; PDMS, polydimethyl-
siloxane; PNA, peptide nucleic acid; PS, phosphate solution; PSA, prostate specific antigen; RNA, ribonucleic acid; RT-PCR, reverse
transcription-polymerase chain reaction; SiNW, silicon nanowire; TnI, troponin I; VGCC, voltage-gated Ca2+ channel; VLS, vapor—liquid—solid.
∗ Corresponding author at: Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan.
Tel.: +886 2 2366 8238; fax: +886 2 2362 0200.
E-mail address: ytcchem@ntu.edu.tw (Y.-T. Chen).
1 These authors contributed equally to this work.
1748-0132/$ — see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.nantod.2011.02.001
132 K.-I. Chen et al.
Introduction ceed the analysis, an electrochemical biosensor provides
an attractive platform to analyze the contents of biologi-
Quantification and analysis of biological processes are of cal samples because of the direct conversion of biological
utmost importance for biomedical applications and cellu- events to electronic signals (that can be detected directly),
lar programming investigation. However, it is challenging thus allowing more rapid and convenient sensing detection.
to convert the biological information into an electronic Investigations of the materials and methods to con-
signal due to the difficulties of connecting an electronic struct an electrochemical biosensor have been underway
device into a biological environment. In recent years, there for decades. Over the past 20 years, nanomaterials, such
has been dramatic development of electrochemical biosen- as quantum dots, nanoparticles, nanowires, nanotubes,
sors because of their applications in toxicity testing [1], nanogaps, and nanoscale films [7—13], have received enor-
chemical analysis [2], medical diagnosis [3], food indus- mous attention due to their suitable properties for designing
try [4], environmental monitoring, and many other areas. novel nanoscale biosensors. For example, the dimension of
An electrochemical biosensor, as defined by IUPAC, is a nanomaterials of ∼1—100 nm provides a perfect feature to
self-contained integrated device that allows for specific ana- study most biological entities, such as nucleic acids, pro-
lytical detection by using a biological recognition element teins, viruses, and cells (as illustrated in Fig. 1(b)) [14].
(a biochemical receptor) in direct spatial contact with a In addition, the high surface-to-volume ratios for nanoma-
transduction element (Fig. 1(a)) [5,6]. Different from a bio- terials allow a huge proportion of the constituent atoms
analytical system (e.g., immunoprecipitation usually used in the material to be located at or close to the sur-
for protein analysis) that requires a reagent addition to pro- face. This characteristic makes the surface atoms play
Figure 1 (a) The construction of typical biosensors with elements and selected components. The procedures are described as
follows: (i) receptors specifically bind the analyte; (ii) an interface architecture where a specific biological event takes place and
gives rise to a signal recorded by (iii) the transducer element; (iv) computer software to convert the signal into a meaningful physical
parameter; finally, the resulting quantity is displayed through (v) an interface to the human operator. (b) The sizes of nanomaterials
(NW and NT) in comparison to some biological entities, such as bacteria, viruses, proteins, and DNA.
Reprinted from [6,14].
Silicon nanowire field-effect transistor-based biosensors for biomedical diagnosis and cellular recording investigation 133
an extremely important role in determining the physical,
chemical, or even electronic properties of nanomaterials.
Moreover, some particular nanomaterials with surfaces that
are easily chemically modified have made them significant
candidates for nanoscale sensing applications.
To date, a variety of nanoscale sensing techniques have
been used for biological research and applications. In par-
ticular, when monitoring living systems, requiring rapid and
precise detection, the demands of sensor architectures
become challenging. Several essential factors, such as ultra-
sensitivity, specificity, high-speed sample delivery, and low
cost must be considered when designing and fabricating
nanoscale biosensors. Some sensing devices selecting quan-
tum dots as their sensing elements possess the merits of high
sensitivity, selectivity, and short response time. However,
this kind of sensing technique generally requires integra-
tion with optical instruments to translate the successful
binding phenomena into a readable signal [7], making the
sensing measurements costly. On the other hand, devices
like field-effect transistors (FETs) can be suitable candidates
for designated sensors, owing to their ability to directly
translate the interaction with target molecules taking place
on the FET surface into a readable signal [15]. In recent
years, one-dimensional semiconducting nanomaterials, such
as silicon nanowires and carbon nanotubes, configured with
FETs (referred to as SiNW-FET [16,17] and CNT-FET [18—20],
respectively) have attracted great attention because they
are an ideal biosensor with high selectivity and sensitivity,
real-time response, and label-free detection capabilities. In
this review article, we will mainly focus on the device fab-
rication of SiNW-FETs and their applications in biomedical
diagnosis and cellular research.
Field-effect transistor-based biosensors
From the electrochemical point of view, SiNW-FET-based
biosensors are a three-electrode system, including source,
drain, and gate electrodes. The function of the source
and drain electrodes is to bridge the semiconductor chan-
nel made of SiNWs and the gate electrode is responsible
for modulating the channel conductance. In a representa-
tive NW-FET example illustrated in Fig. 2(a), the biological
receptors were anchored to the surface of the semiconduc-
tor channel by chemical modification to recognize the target
analytes through their high specificity and strong binding
affinity in the buffer environment. The target—receptor
carriers, causing an increase in conductance. (c) Schematic
representation of a CNT-FET device including the surface modi-
Figure 2 (a) The illustration of a nanoscale FET biosensor fication and molecular recognition procedures: (1) modification
with a cross-sectional view. The semiconductor channel (NW or of linkers onto the single-walled CNT through — interac-
NT) is placed between the source and drain electrodes with a tion; (2) immobilization of antibody; (3) detection of antigen by
gate electrode on the bottom to modulate the conductivity of antibody. (d) CgA was released from neurons stimulated by glu-
the semiconductor channel. Target molecules can be recognized tamate and was detected by CgA-Ab/CNT-FET. A coverslip with
by the receptor modified on the channel surface through strong grown neurons was positioned on the CgA-Ab/CNT-FET device
binding affinity. (b) When positively charged target molecules with neurons facing the FET circuits. Immediately after the glu-
bind the receptor modified on a p-type NW, positive carriers tamate (50 M) stimulation, a dramatic increase in current was
(holes) are depleted in the NW, resulting in a decrease in con- detected due to the binding of the released CgA to CgA-Ab/CNT-
ductance. On the contrary, negatively charged target molecules FET.
captured by the receptor would make an accumulation of hole Reprinted from [14,38].
134 K.-I. Chen et al.
interaction then varied the surface potential of the semi- mechanisms of a CNT-FET are somewhat complex and were
conductor channel and modulated the channel conductance, reported to involve field-effects [18,40], electron transfer
and the signal was eventually collected by a detection sys- [18], Schottky barriers [41,42], etc. In contrast, the sens-
tem. ing mechanism of a SiNW-FET sensor is straightforward and
A diversity of FET-based biosensors has been employed simply dominated by the field-effect [16,43] due to the
for biological applications. Here, we tried to classify these interaction between the target analyte and the receptor
biosensors into enzyme-modified FETs, cell-based FETs, and modified on the surface of the SiNW-FET.
immunologically functionalized FETs. Enzyme-modified FETs
comprise a redox active enzyme integrated with an elec-
tronic circuitry to give a real-time quantitative analysis
Silicon nanowire field-effect transistors
of the enzyme substrate [21], e.g., sensing glucose from
a catalytic reaction in the presence of glucose oxidase. Taking advantage of the well-developed silicon industry,
Cell-based FETs were exploited to detect the released bio- SiNW-FETs can benefit from existing and mature silicon
chemical agents or real-time cellular responses from living industry processing techniques and fabrications. In the syn-
cells, such as action potentials from neuron cells [22] or thetic reactions that prepare SiNWs, different sizes [44,45],
electrical recordings from chicken hearts [23]. shapes [46], and dopants [47] of SiNWs could be precisely
In general, immunologically functionalized FETs are the tailored. Because SiNWs could be well-controlled during the
most frequently used biosensors. For example, an antibody- wire growth, the performance exhibits high reproducibility.
modified FET sensor can be used to detect the corresponding Therefore, the n-/p-type semiconducting property, doping
antigen. Depending on the charge carriers in the semicon- density, and charge mobility in a SiNW-FET can be designed
ductive channel (holes for a p-type channel and electrons for in advance. In the following sections, we selectively dis-
an n-type channel), the direction of the conductance change cuss the nanowire fabrication, assembly techniques, device
represents the sign of the charges carried by the target anti- array design, and electrical measurement setup used in the
gen, and the magnitude of the conductance change reflects performance of SiNW-FETs.
the antigen—antibody interaction. In an example of a p-type
NW-FET illustrated in Fig. 2(b), when the positively charged Fabrication of SiNW-FETs
analytes bind the receptor-anchored NW-FET, a depletion of
charge carriers occurs in the conductance channel, causing There are two major fabrication techniques in preparing
a decrease in the device conductivity. On the contrary, an SiNW-FETs: ‘‘top-down’’ and ‘‘bottom-up’’. The ‘‘top-
increase in the device conductivity would result from the down’’ method is carried out through lithographic processes
accumulation of charge carriers in the conductance channel combined with an electron-beam technique that defines
while negatively charged molecules, such as DNA or RNA, SiNW-FETs by physically etching a single-crystalline silicon
bind the p-type NW-FET. wafer [48]. On the other hand, the ‘‘bottom-up’’ processes
In recent years, many types of semiconducting materials, start with the growth of SiNWs, normally in a chemical vapor
such as carbon materials (e.g., CNT and graphene) [24—26] deposition (CVD) reaction, followed by SiNW assembly and
and metal-oxide nanowires (e.g., In2 O3 -NW and ZnO-NW) electrode fabrication via the photolithographic or electron-
[27,28], have been selected as promising candidates for beam lithographic procedures [43].
the development of FET-biosensors. For instance, graphene-
based FETs were constructed for electrically detecting
pH values, bovine serum albumin adsorption [25], and ‘‘Top-down’’ SiNW-FETs
cellular recording [26]. In2 O3 -NW [27] and ZnO-NW [28]
configured with FET-biosensors were also used to monitor The ‘‘top-down’’ method for the SiNW-FET fabrication is
protein—protein interactions. Among them, CNTs, singled- based on lithographic processing on a silicon-on-insulator
walled CNTs in particular, were at the forefront of these (SOI) wafer. As illustrated in Fig. 3(a, i), the structure
explorations. Several recent articles about CNT-FETs, as of an SOI wafer contains three layers: substrate Si wafer,
represented in Fig. 2(c), have reviewed their biological buried silicon dioxide (about 200—400 nm thick), and top Si
applications [29—31], such as antigen—antibody interactions layer (about 50—100 nm thick). Through the standard pro-
[27,32—34], DNA hybridization [35,36], and enzymatic glu- cedures of photolithography, reactive ion etching (RIE), ion
cose detection [37]. As depicted in Fig. 2(d), a CNT-FET implantation, electron-beam lithography, and thermal evap-
was specifically applied to the real-time detection of a oration, SiNWs and the connecting electrodes can be defined
cancer marker for neuroendocrine tumors, namely chromo- to form SiNW-FET devices, in which the width of SiNWs could
granin A (CgA), released from embryonic cortical neurons reach the scale of ∼100 nm.
[38,39]. The CNT-FET device modified with the antibody of A typical ‘‘top-down’’ process to fabricate SiNW-FETs is
CgA (referred to as CgA-Ab/CNT-FET) was employed to mon- illustrated schematically in Fig. 3(a) [49—56]. In Step 1, the
itor the in situ release of CgA from living neurons in response Si layer is doped with low-density boron or phosphorous
to glutamate stimulation. of ∼1015 /cm3 (about 10—20 cm). Therefore, the n-/p-
Despite these advances of CNT-FETs in biosensory appli- type semiconducting property and doping ratio of SiNWs are
cations, several shortcomings were encountered in the determined (Fig. 3(a, i)). Step 2 is to define the source and
fabrication and applications of CNT-FETs. First, in the fab- drain leads with heavy doping (1019 /cm3 ), of which the pat-
rication of CNT-FETs, the mixtures of semiconducting and terns are drawn with a photomask design (Fig. 3(a, ii)). In
metallic CNTs still hamper future developments in nanoelec- Step 3, the micrometer-sized source and drain electrodes
tronics. Secondly, the determining factors for the sensing are finished by RIE etching (Fig. 3(a, iii)). The following
Silicon nanowire field-effect transistor-based biosensors for biomedical diagnosis and cellular recording investigation 135 Figure 3 (a) Schematic illustration of a typical ‘‘top-down’’ process to fabricate SiNW-FETs. (i) In Step 1, the silicon layer is doped with low-density boron or phosphorous of ∼1015 /cm3 . (ii) In Step 2, specific regions defined with a photomask pattern receive heavy doping (1019 /cm3 ). (iii) In Step 3, the micrometer-sized source and drain electrodes are finished by RIE etching. (iii) The following Step 4 is to fabricate the nanometer-sized SiNWs with an electric-resist pattern and RIE etching. (b) An illustration of a ‘‘bottom- up’’ method to fabricate SiNW-FETs. (i) The growth of SiNWs in CVD reaction via the VLS mechanism. (ii) Deposition/alignment of SiNWs on a silicon substrate. (iii) A photomask pattern to define source/drain electrodes. (iv) Thermal evaporation to deposit the source/drain contacts. (v) Lift-off the remaining photoresist with Remover PG. Step 4 is to fabricate the nanometer-sized SiNWs with an ‘‘Bottom-up’’ SiNW-FETs electric-resist pattern and RIE etching (or tetramethylam- monium hydroxide etching [49]) (Fig. 3(a, iv)). Subsequently, The ‘‘bottom-up’’ processes start with the growth of SiNWs, a thermal evaporation is used to make the contact leads normally in a chemical vapor deposition (CVD) reaction, fol- and back-gate, and finally an insulator layer (e.g., Al2 O3 lowed by SiNW assembly (assisted by various techniques [49,53], SiO2 [51], or Si3 N4 [56]) is coated on the SiNW-FET that are discussed in the next section), and finally the devices. device fabrication via the photolithographic or electron- Compared with the ‘‘bottom-up’’ method, the ‘‘top- beam lithographic procedures [43]. With the ‘‘bottom-up’’ down’’ approach is more complex because the process method, SiNWs can be synthesized catalytically in a CVD relies on high-resolution lithography. For this reason, reaction via the vapor—liquid—solid (VLS) growing mecha- electron-beam lithography is necessary. Although the ‘‘top- nism (Fig. 3(b, i)) [57]. The synthesis is usually catalytically down’’ approach needs many luxurious instruments, it has assisted with metal nanoparticles [58,59] that not only cat- advantages of using standard semiconductor techniques to alyze the SiNW formation, but also control the size of precisely design a desired device-array pattern without the as-synthesized SiNWs. Subsequently, the as-synthesized problems of positioning SiNWs. Another challenge to the SiNWs are suspended in ethanol solution and dispersed onto ‘‘top-down’’ method is that the minimum width of the pro- a support silicon substrate (Fig. 3(b, ii)). In the following duced SiNWs is around 100 nm. To overcome this barrier, photolithographic steps, a two-layer photoresist consisting single SiNWs of triangular section were fabricated to reach of LOR3A and S1805 was first deposited onto a silicon sub- the transverse dimension of ≤20 nm with the length of sev- strate by spin coating where the electrodes were defined eral micrometers [55]. with a photomask design (Fig. 3(b, iii)). The next step is
136 K.-I. Chen et al.
to deposit metal for the source/drain contacts by thermal ment of the applied gate voltage (Vg ) vs. source-drain
evaporation (Fig. 3(b, iv)). Finally, the remaining photoresist voltage (Vsd ), Freer et al. reported that single SiNWs
layer was lifted off by Remover PG (Fig. 3(b, v)) [43]. Com- could be assembled over 98.5% of 16,000 pre-patterned
pared with the ‘‘top-down’’ technique, the ‘‘bottom-up’’ electrode sites through controlling the balance of surface,
method has the advantages of synthesizing SiNWs of high hydrodynamic, and dielectrophoretic forces (Table 1(d))
crystallinity, designated dopant density, thin silicon oxide [66].
sheaths, and easily controlled diameters in a cost-effective Smearing-transfer method. The smearing-transfer (or
preparation. However, without a deliberate alignment for contact-printing) method is one of a series of alignment
the randomly orientated SiNWs on the silicon substrate, methods developed by Ali Javey’s group. This method is
the device fabrication would suffer from inefficient fabrica- based on a direct contact printing process that enables the
tion yields, which could also limit their development in the direct transfer and positioning of SiNWs from a donor sub-
industrial applications. Therefore, the success of producing strate to a receiver chip. This simple method can efficiently
high-quality SiNW-FETs calls for developing a uniform assem- transfer a variety of NWs (such as SiNWs and Ge-NWs) to a
bly of the ‘‘bottom-up’’ synthesized SiNWs on the support wide range of receiver substrates, including silicon and flex-
substrates. ible plastics. The technique actually uses chemical ‘‘lawn’’
and ‘‘lubricant’’ to increase the density and alignment qual-
ity and can be regarded as a rapid, efficient, and economic
Nanowire assembly techniques method (Table 1(e)) [70].
Significant efforts have been invested in developing generic Roll-printing assembly. On the basis of the contact-
methods to align NWs for the device assembly to fab- printing process, Ali Javey’s group also developed an
ricate NW-FETs. Several techniques for the assembly of approach for a scalable and large-area printing. Roll-to-roll
NWs have been achieved, including flow-assisted alignment assembly has made it possible to produce highly ordered,
[60], Langmuir—Blodgett technique [61—64], bubble-blown dense, aligned, and regular arrays of NWs with high unifor-
technique [65], electric-field-directed assembly [66—69], mity and reproducibility using differential roll printing. The
smearing-transfer method [70], roll-to-roll printing assem- schematic of the differential-roll-printing system setup and
bly [71], and polydimethylsiloxane (PDMS) transfer method the results of the wire alignment are shown in Table 1(f).
[72,73]. The optical and scanning electron microscopy (SEM) images
Flow-assisted alignment. In the flow-assisted align- of the roller printed Ge-NWs on a Si/SiO2 substrate clearly
ment method (Table 1(a)), the suspended SiNWs were indicate the well-aligned and dense (about 6 NWs/m) NW
passed through the microfluidic channel structures formed parallel arrays. The differential-roll-printing process is com-
between a PDMS mold and a flat SiO2 /Si substrate patible with the smearing-transfer method and can also be
[60]. The SiO2 /Si substrate was pre-modified with 3- implemented in a wide range of rigid and flexible substrates
aminopropyltriethoxysilane (APTES), of which one end is [71].
anchored to the SiO2 surface and the other end forms an PDMS-transfer method. Chang et al. developed a NW align-
NH2 -terminated surface. This NH2 -terminated surface will ment method using a PDMS stamp (Table 1(g)) [72]. In the
help the alignment of SiNWs via electrostatic interactions. report, a high-speed roller (20—80 cm/min) was used to
While the angular spread of the SiNWs in the flow direction assist the transfer of ZnO-NWs from the growth substrate
is flow-rate dependent, the density of the SiNWs assembled to a PDMS stamp; these NWs were then re-transferred from
on the SiO2 /Si substrate is time dependent. the PDMS stamp to another receiver substrate. With this
Langmuir—Blodgett technique. The Langmuir—Blodgett method, NWs can be aligned with high density, providing
technique can be applied for the alignment of NWs/NTs. As a convenient and efficient approach for the fabrication of
shown in Table 1(b), this solution-based method assembles NW-FETs.
SiNWs in a monolayer of surfactant at the air—water inter-
face and then compresses the SiNWs on a Langmuir—Blodgett Array design and electrical measurement setup
trough to a specified pitch. The aligned SiNWs are then The SiNW-FET devices could be fabricated following a
transferred to the surface of a substrate to make a uni- standard photolithographic procedure with a mask design
form parallel array. Crossed SiNW structures could further depicted in Fig. 4(a). The synthesized SiNWs were dispersed
be formed by uniform transfer of a second layer of aligned on a SiO2 /Si substrate (typically 400 nm-thick SiO2 ). The
parallel SiNWs perpendicular to the first layer [61—64]. as-dispersed SiNWs in the central area (the reddish rect-
Compared with other methods, the Langmuir—Blodgett angles in Fig. 4(a) and (b)) were electrically connected by
technique can prepare an ultrahigh-density SiNW alignment. metal leads (represented in yellow in Fig. 4(a)). The sur-
Bubble-blown technique. The bubble-blown technique is faces of the metal electrodes were further coated with an
another physical assembly method (Table 1(c)), in which the insulating layer to prevent electric leakage during sens-
SiNWs were suspended in tetrahydrofuran solution and then ing experiments. The bottom inset graph enlarged from
blown into a single bubble using a nitrogen flow to form SiNW the red region in Fig. 4(b) displays the array design, in
blown-bubble films [65]. The uniqueness of this method is which the individual nanowire device was connected by
that blown-bubble films can be transferred to both rigid and metal electrodes with a separation of several micrometers.
flexible substrates during the expansion process. It can also The individual SiNW situation can be seen with the image
be scaled to large wafers and non-rigid substrates [65]. scanned by atomic force microscope (AFM), as shown in
Electric-field-directed assembly. The electric-field- Fig. 4(c).
directed assembly of SiNWs is an intriguing and desirable The experimental setup involved in electrical measure-
method. From the appropriate electrode design and adjust- ments includes a silicon chip (1.5 mm × 1.5 mm) containing
Silicon nanowire field-effect transistor-based biosensors for biomedical diagnosis and cellular recording investigation 137 Table 1 Nanowire assembly techniques. (a) Schematic and results of a flow-assisted NW assembly method. The suspending NWs were passed through a fluidic channel resulting in the alignment of NWs on a flat substrate, but often with low NW density. (b) A flow-assisted NW Langmuir—Blodgett assembly method. This method involves packing aligned NWs in a monolayer of surfactant at the air—water interface, followed by transferring to the surface of a substrate. The scalability and uniformity of alignment for large-scale is still challenging; however, the assembled NWs are highly aligned and of high density. (c) A bubble-blown NW assembly method. The NWs were suspended in tetrahydrofuran solution and then blown into a single bubble using a nitrogen flow to form the SiNW blown-bubble films, followed by transferring to the surface of a substrate by direct contact. Although this approach cannot align NWs to be of high density, it has advantage of fitting the receiver substrate in different shapes. (d) An electric-field NW assembly method. The alignment is induced by polarizing NWs in an applied electric field. Under a delicate control, the ratio for a successful alignment could be over 90%. (e) A smearing-transfer NW assembly method. This method is based on a direct contact-printing process that enables direct transferring and positioning of NWs from a donor substrate to a receiver chip. This method can be employed for large scale NWs transfer with high alignment and density. (f) A roll-printing NW assembly method. This method applies a glass roller as the substrate for NWs growth, and then uses this roller as a NWs donor to transfer NWs to a receiver substrate through shear force. (g) A PDMS-transfer NW assembly method. This method is an amboceptor in the NWs transfer process, which adheres NWs by stamping the donor substrate and transferring NWs to another receiver substrate. Although the directions of aligned NWs were not perfect, it is a convenient method with a potential to be developed further in the future. (a) Flow-assisted [60] (b) Langmuir—Blodgett [61] (c) Bubble-blown [66] (d) Electric-field [65] (e) Smearing-transfer [70] (f) Roll-printing [71]
138 K.-I. Chen et al.
Table 1 (Continued )
(g) PDMS-transfer [72]
Reprinted from [60,61,65,66,70—72].
SiNW-FET device arrays, a PDMS microfluidic channel GSH/GST-tag
(6.25 mm × 0.5 mm × 0.55 mm), and a detection system.
First, the silicon chip containing SiNW-FET device arrays The reversible binding between glutathione S-transferase
was mounted on a plastic circuit board and electrically con- (GST) and glutathione (GSH) has long been applied in pro-
nected with ∼30 m-diameter aluminum wires (Fig. 4(d)) tein purification. Through molecular cloning techniques,
before electrical measurement. The PDMS microfluidic the GST sequence can be incorporated into an expression
channel was then placed in the middle of the chip to vector alongside the gene sequence encoding the pro-
allow sample solution delivery onto the SiNW-FET arrays tein of interest. Thus, various GST-fusion proteins can be
(Fig. 4(e)). The detection system including a lock-in easily produced in a large scale via bacterial or mam-
amplifier and a current pre-amplifier was to record the malian expression systems. By using GSH-conjugated resins
electrical signals resulting from the binding events occur- to trap GST recombinant protein from whole cell extract
ring on the SiNW-FET surface during sensing experiments and then washing the resins with buffer to remove con-
(Fig. 4(f)). taminating bacterial or mammalian proteins, the pure
It is noted that the laminar flow in an ordinary microflu- GST-fused protein can be eluted easily by a high concentra-
idic channel used in FET-based measurements may restrict tion of GSH. Taking advantage of the reversible GSH—GST
the detection sensitivity due to the diffusion-limited sample association—dissociation, Lin et al. adapted this method
delivery [74]. Comparatively, a specially designed micro- to make SiNW-FET a reusable biosensor [51]. As illustrated
scale solution chamber with efficient sample mixing during schematically in Fig. 5(a), a SiNW-FET was first modified with
the fluid exchange has been demonstrated to improve the GSH (referred to as GSH/SiNW-FET) and then anchored with
detection sensitivity [49,75]. a particular GST-fused protein (referred to as protein-GST).
This protein-modified SiNW-FET could then be employed
to screen possible interacting proteins. After the sensing
Reusable SiNW-FET system measurements of protein—protein interactions, the used
protein-GST on the GSH/SiNW-FET could be easily removed
In the application of SiNW-FETs for the biomedical diagnosis with a GSH (≥1 mM) washing solution. The reversible
of a particular target (e.g., an antigen), the correspond- GSH—GST association—dissociation has made the SiNW-FET
ing receptor (e.g., the antibody) is usually modified on sensorial device reusable and calibratable, thus allowing
the SiNW-FET surface prior to the detection. By virtue for quantitative analysis in sensing measurements. This
of the strong and specific binding affinity between anti- biologically modified SiNW-FET can be applied as an ultra-
gen and antibody under normal physiological conditions, sensitive biosensor for fast high-throughput screening of
the receptor-modified SiNW-FET can serve as an extremely biomolecular associations, such as protein—protein interac-
sensitive sensor with high selectivity. By the same token, tions, protein—DNA interactions, and protein—carbohydrate
because of this strong binding between the antigen and interactions. Very recently, Lin et al. also applied this tech-
antibody, it is difficult to remove the antigen—antibody nique of using a reusable SiNW-FET to detect the interactions
complex from the surface of SiNW-FET after detection, of calmodulin with purified cardiac troponin I (∼7 nM) and
meaning that a SiNW-FET could be used only for a single crude N-type Ca2+ channel extracts [76].
measurement. With this limitation, consecutively quan-
titative analysis by a calibratable SiNW-FET is hard to Ni2+ /His6 -tag
achieve.
To solve this problem, several reversible surface modi- The polyhistidine-tag is an amino acid motif that consists
fication techniques have been developed recently, leading of multiple histidine (His) residues at the N- or C-terminus
to reusable SiNW-FET devices. Two well-known protein of the protein. The total number of His residues may vary,
trapping systems wildly used in protein purifications, but there are normally six in the tag; therefore, it gen-
the GSH/GST-tag [51,76] and Ni2+ /His6 -tag [77—79], were erally named a His6 -tag. Similar to the GST-tag system,
adapted to serve as a reversible surface modification method the His6 -tag is a popular and efficient system for puri-
on SiNWs and will be discussed in the following section. In fying proteins via the reversible association—dissociation
addition, the application of a cleavable disulfide bond that between the His6 -tag and the affinity resins; the associa-
served as a linker between the receptor and a SiNW-FET has tion is assisted with metal ions, either nickel or cobalt. The
also been reported for use as a reusable SiNW-FET system reversible immobilization of His6 -tagged proteins to a sensor
[49]. surface was recently applied to CNT-FET [77] and demon-
Silicon nanowire field-effect transistor-based biosensors for biomedical diagnosis and cellular recording investigation 139 Figure 4 (a) Mask design for the photolithographic fabrication of SiNW-FET device arrays. (b) Device arrays on a magnified scale. Top: Optical image of the circuits in the area of the yellow square in (a); bottom: SEM image of a SiNW-FET array with a source-drain separation of 2 m. The scale bar is 50 m. (c) The topograph of a SiNW-FET scanned by AFM. A SiNW of ∼50 nm in diameter is connected by two Ni/Al (70 nm/100 nm in thickness) electrodes of ∼2 m in separation. (d) The SiNW-FET device arrays on a silicon chip (1.5 mm × 1.5 mm) are connected to a plastic circuit board with aluminum wires (∼30 m in diameter). (e) A sample solution was delivered onto the SiNW-FET arrays through a PDMS microfluidic channel (6.25 mm × 0.5 mm × 0.05 mm), which was designed to couple with the SiNW-FET device arrays. (f) The variation of electrical signals was monitored by a detection system that combined a lock-in amplifier and a current preamplifier. Reprinted from [38]. strated on SiNW [78]. As demonstrated in Fig. 5(b), the protein immobilization, resulting in the retrieval of the FET hexavalent Ni2+ ions held by the nitrilotriacetic acid (NTA) surface. In comparison with the GSH/GST-tag system, the chelator groups were chemically modified to the FET surface smaller Ni2+ /His6 -tag has its advantages in the FET-based and then bound to His6 -tagged protein through the coordi- sensing measurements. First, because of its smaller size, nation between His residues and the remaining two sites more Ni2+ /His6 -tags could be anchored on the FET surface, of the hexavalent Ni2+ ions. The addition of imidazole or thus increasing the sensing sensitivity. Secondly, the binding ethylenediaminetetraacetic acid (EDTA) can compete with sites located on the smaller-sized Ni2+ /His6 -tag are closer the His—metal interaction to cause a reversed process of the to the FET surface, resulting in a lesser screening effect
140 K.-I. Chen et al.
Figure 5 A schematic illustration for the reversible SiNW-FET system. (a) A SiNW-FET is first modified with 3-aminopropyl-
trimethoxysilane (APTMS) and 3-maleimidobenzoic acid N-hydroxysuccinimide ester (MBS) linkers, then functionalized with GSH
to form GSH/SiNW-FET. A particular GST-fusion protein (referred to as protein-GST) is anchored on the GSH/SiNW-FET via the
GST—GSH association. The protein-immobilized SiNW-FET is then employed for screening possible interacting proteins. At the end
of each measurement, the used protein-GSTs are removed with GSH (≥1 mM) washing solution, making the GSH/SiNW-FET a reusable
biosensor. (b) The Ni2+ /His6 -tag functionalized sensor surface was formed by (3-glycidyloxypropyl) trimethoxysilane (GPTMS) and
N-(5-amino-1-carboxypentyl) iminodiacetic acid (AB-NTA). His6 -tagged proteins can then be trapped on the sensor surface. After
the measurement, the His6 -tagged proteins can be removed by imidazole to retrieve the sensor surface.
Reprinted from [51,78,79].
on the FET detections, which also increases the sensing affected by the electric field exerted from the charged par-
sensitivity. ticles is only located at or close to the wire surface. Namely,
the interior areas of the wires could still be unaffected.
In sharp contrast, as the wire diameter decreased, say to
Sensing measurements nanoscale, the surface-to-volume ratio increases drastically
and the influence of the external electric field could reach
Size effect on sensing sensitivity the whole cross-section of the NW. As such, the induced
conductance change inside the NW-FET could overwhelm-
The wire size of a SiNW-FET can also affect the sensitivity ingly prevail over microwire-FETs [6,54]. Elfstrom et al.
of the FET device. As illustrated in Fig. 6, the surface-to- have demonstrated the size-dependent sensitivity of SiNW-
volume ratio of thick wires is relatively small (Fig. 6(a)) FETs [54]. When SiNW-FETs of different wire widths are
compared to that of thin wires (Fig. 6(b)). Therefore, when immersed in an acidic buffer solution, the FET devices con-
thick wires are approached by charged particles, the area figured by smaller diameter SiNWs exhibit large conductanceSilicon nanowire field-effect transistor-based biosensors for biomedical diagnosis and cellular recording investigation 141
Figure 6 An illustration for the concept of a size effect on the
conductance change in a wire. (a) For thick wires, the surface-
to-volume ratio is relatively small. When the wire surface is
approached by charge particles (red ball), only the conductance
near the wire surface is affected. There is still a large interior
area of the wire that might not be influenced (gray circle). (b)
As the wire diameter is reduced to nanoscale, the surface-to-
volume ratio drastically increases. Therefore, the same external
electrical field (pink area) caused by the charge particles (red
ball) could influence most of the interior area of the NW, thus
drastically changing its conductance.
Reprinted from [6].
changes, whereas those of larger diameter SiNWs remain
unaffected.
Figure 7 (a) A schematic showing the height of D from the
sensor surface for an electrolytic buffer solution. The horizontal
Debye—Hückel screening dashed lines mark the heights of D = 0.7, 2.4, and 7.4 nm for 1×
PBS (blue), 0.1× PBS (red), and 0.01× PBS (black), respectively.
In the FET-based biosensing measurements, the solution (b and c) Real-time electrical measurements of the association
environment plays an important role in determining the and dissociation of GST on a GSH/SiNW-FET in (b) 0.1× PBS
sensing performance. In order to create a surrounding sim- (black curve, D = 2.4 nm) and 1× PS (red curve, D = 1.9 nm)
ilar to a normal physiological environment, like human and (c) 0.01× PBS (black curve, D = 7.4 nm) and 0.1× PS (red
serum or urine, phosphate buffered saline (1× PBS, 137 mM curve, D = 6.1 nm).
NaCl, 2.7 mM KCl, 10 mM Na2 HPO4 , 2 mM KH2 PO4 , pH 7.4 Reprinted from [51].
with NaOH) or phosphate solution (1× PS, 2.4 mM NaH2 PO4 ,
7.6 mM Na2 HPO4 , pH 7.4 with NaOH) was generally selected
as the matrix, in which analytes dissolved during the mea- where ε0 represents the vacuum permittivity, εr is the
surements. However, in solutions containing such high-salt relative permittivity of the medium, kB is the Boltzmann
concentrations, the interaction potential (V(r)) between the constant, T represents the absolute temperature, NA is Avo-
receptor and analytes that cause the conductance change gadro’s number, e stands for the elementary charge, and I
in the FET sensor could be partially screened by the strong represents the ionic strength of the electrolytic buffer solu-
ionic strength of the electrolytic buffer solution, thus reduc- tion. It is obvious that an electrolytic solution of higher ionic
ing the signals obtained from the electrical measurements. strength (I) has a shorter D , thus creating a more severe
The screening of V(r) in the FET measurements is enhanced screening effect on the FET-based sensing measurements.
exponentially by the distance (rbs ) measured from the bind- Calculations from Eq. (2) give D = 0.74 nm for the 1× PBS
ing site of receptor—analyte complex to the FET surface and solution and D = 1.94 nm for the 1× PS solution.
can be represented as As represented schematically in Fig. 7(a), depending on
V (r)e−r/D at r = rbs (1) the rbs value in a FET measurement, the electrolytic buffer
solution needs to be properly selected to be of an appropri-
where D is the Debye—Hückel length [80,81] and is given by ate D without jeopardizing the signal collection. In Fig. 7(b)
and (c), the screening effect due to the electrolytic buffer
ε0 εr k B T solution was experimentally demonstrated from the bind-
D = (2)
2NA e2 I ing of GST to GSH/SiNW-FET [51], where the conductance142 K.-I. Chen et al. Figure 8 (a) Real-time electrical measurements of the association of CaM-GST with a GSH/SiNW-FET in 0.1× PS solution supplemented with 0.5 mM EDTA (pH 7.4). The arrow indicates the arrival of the CaM-GST solution. (b) Real-time detection of the binding of K+ (red), Al3+ (green), and Ca2+ (blue) to CaM/SiNW-FET. The arrow indicates the arrival of the appropriate ion solution. (c) Real-time detection of the binding of cardiac TnI to CaM/SiNW-FET in 0.1× PS solution supplemented with 100 M Ca2+ . (d) Specificity of CaM/SiNW-FET. The electrical conductance of CaM/SiNW-FET showed no response until binding of TnI. (e) Plot of G vs. log[TnI]. The addition of various concentrations of TnI in 0.1× PS supplemented with Ca2+ (square) or 0.5 mM EDTA (cir- cle). The red line represents a linear fit to the five concentration data points (correlation coefficient = 0.987). (f) Real-time electrical
Silicon nanowire field-effect transistor-based biosensors for biomedical diagnosis and cellular recording investigation 143
change (G) of the GSH/SiNW-FET in measuring 15 nM GST tion lately for studying protein interaction mechanisms, not
(Fig. 7(c)) in 0.01× PBS (black curve, D = 7.4 nm) or 0.1× PS only because of its real-time and label-free detection, but
(red curve, D = 6.1 nm) is enhanced roughly fourfold com- also due to its high sensitivity and selectivity. An early mea-
pared to the measurements (Fig. 7(b)) in 0.1× PBS (black surement made by Cui et al. demonstrated the real-time
curve, D = 2.4 nm) or 1× PS (red curve, D = 1.9 nm). detection of streptavidin binding to biotin-modified SiNW-
Gao et al. have recently reported that the optimal sen- FET [88]. They also explored the ability of biotin-modified
sitivity of a SiNW-FET in biosensing measurement can be SiNW-FET to detect streptavidin at the concentration of
achieved by judiciously selecting the subthreshold regime, 10 pM, which is much lower than the nanomolar-range detec-
where the gating effect from target molecules is most effec- tion level obtained from other techniques, such as the
tive due to the reduced screening of carriers inside the SiNW stochastic sensing of single molecules [89].
[82]. The effectiveness of gating effect induced by target In addition to the biotin—streptavidin investigation, the
molecules at the surface of a SiNW-FET sensor is determined concept of detecting protein—protein interactions with
by the relative magnitude between carrier screening length SiNW-FET could be extended to broad applications. A
(Si ) and SiNW radius (R). In the high carrier concentration calmodulin (CaM)-modified SiNW-FET sensor has recently
regime (Si R), SiNW-FET works in a linear regime and the been used to detect calcium ions (Ca2+ ) by Lin et al. [76];
conductance varies with gate voltage linearly. In the low their experiments also showed that Ca2+ -bound CaM is able
carrier concentration regime (Si R), the SiNW-FET works to activate various proteins involved in physiological activ-
in the depletion (subthreshold) regime and the conductance ities, such as the binding between Ca2+ -bound CaM and
varies with gate voltage exponentially. It is demonstrated cardiac troponin I (TnI). CaM was anchored to a reusable
that the most sensitive SiNW-FET biosensor should utilize SiNW-FET (referred to as CaM/SiNW-FET) via the afore-
the field gating effect of surface charges throughout the mentioned GSH—GST association—dissociation. As shown in
whole cross-section of SiNW, which requires Si > R. In the Fig. 8(a), a dramatic increase in conductance verified the
subthreshold regime of a SiNW-FET, carriers in the SiNW have successful binding of negatively charged GST-fused CaM
long screening length (Si > R) and the field effect of surface (referred to as CaM-GST, pI of CaM ∼4 and pI of GST ∼6.72)
charges can gate the whole SiNW, fully utilizing the high to the p-type GSH/SiNW-FET. In order to examine how the
surface-to-volume ratio of SiNW and effectively reaching the binding of various metal ions affects the conductance of
optimal detection sensitivity of the FET sensor. CaM/SiNW-FET, three different metal ions (Ca2+ , Al3+ , and
K+ ) were selected to be examined in this system. As demon-
strated in Fig. 8(b), CaM/SiNW-FET showed a preference for
Applications of SiNW-FET sensors binding Ca2+ (blue curve), but not K+ (red curve) or Al3+
(green curve), according to the conductance change (G)
Protein—protein interaction after the arrival of each metal ion solution. These results
reveal a high specificity of CaM/SiNW-FET for sensing Ca2+ .
A huge number of approaches have been developed Shown in Fig. 8(c) is the binding of the positively charged
to understand molecular complex interactions, such as protein troponin I (TnI, pI ∼9.3) onto CaM/SiNW-FET in 0.1×
protein—protein or protein—small molecule interactions. PS (pH 7.4) containing 100 M Ca2+ , which led to a sizable
For example, a fluorescence detection method combined decrease in the conductance of the FET sensor. The con-
with a fiber-optic biosensor has been established to study trol experiments carried out in Fig. 8(d) reflected that the
the binding kinetics of immunoglobulin G (IgG)/anti-mouse association between CaM and TnI is specific and can only
IgG and human heart-type fatty acid-binding protein (its be triggered in the presence of Ca2+ . It has been proven in
antibody) [83]. However, this labeling detection method was Fig. 8(e) that the G increased with a rising concentration of
limited by some drawbacks. For instance, the surface char- TnI (i.e., log [CaM]) in the presence of Ca2+ . It is also demon-
acteristics of small proteins might be changed after chemi- strated in Fig. 8(f) that the concentration of Ca2+ required
cal labeling, thus varying the labeling efficiency for different to activate the interaction between CaM and TnI was at the
proteins, which consequently makes accurate quantifica- micromolar level (i.e., 10−6 M Ca2+ ).
tion detection difficult. Also, this labeling technique usually In addition, CaM/SiNW-FET was applied to detect Ca2+
requires a huge amount of time for the labeling procedures. channels in cell lysate. The N-type voltage-gated Ca2+
In recent years, some label-free detection techniques, such channels (VGCCs) located at the plasma membrane medi-
as surface plasmon resonance imaging (SPRI) [84], AFM [85], ate the entry of Ca2+ into cells in response to membrane
and SiNW-FET and CNT-FET [86,87] have been invented for depolarization. As illustrated in Fig. 8(g), transfected 293
sensing protein—protein interactions. Among these sensing T cells containing N-type VGCCs resuspended in 1× PBS
approaches, SiNW-FET has attracted more and more atten- were sonicated and then centrifuged to isolate the mem-
measurement for determining the [Ca2+ ] required to activate the interaction between TnI and CaM, where the binding of TnI to
CaM/SiNW-FET was detected at various [Ca2+ ]. The result revealed that the minimal [Ca2+ ] ∼1 M is needed to trigger the CaM
activation. (g) Schematic illustration for the detection of membrane fractions containing N-type VGCCs utilizing CaM/SiNW-FET.
Real-time electrical detections of the binding of N-type VGCCs to CaM/SiNW-FET in 0.1× PS supplemented with (h) 100 M Ca2+ ; (i)
0.5 mM EDTA. (j) (Top graph) Electrical detection of the membrane fraction without the ␣1b subunit by CaM/SiNW-FET in 0.1× PS
supplemented with 100 M Ca2+ and (bottom graph) electrical detection of N-type VGCCs by GST/SiNW-FET in 0.1× PS supplemented
with 100 M Ca2+ .
Reprinted from [76].144 K.-I. Chen et al. Figure 9 (a) A structure of the associated PNA and DNA. (b) Schematic representation showing that the distance from the bound DNA to the SiNW surface could be varied by controlling the location of the DNA—PNA hybridization. (c) Distinguishable resistance changes in the PNA-modified SiNW-FET resulting from the varied hybridization sites measured for two different concentrations of target DNAs. (d) Plot of the experimental ratio of resistance change vs. calculated distance (L) from DNA strands to the SiNW surface. Reprinted from [81]. brane fractions. The solution retreated in 0.1× PS was All of the results outlined above suggest that protein- subsequently introduced into CaM/SiNW-FET; the decreased modified SiNW-FET sensors exhibit high sensitivity and conductance shown in Fig. 8(h) indicates the binding of excellent specificity and are able to detect target molecules VGCCs to CaM/SiNW-FET. On the other hand, an apprecia- rapidly and precisely. This novel technique provides a ble decrease in conductance of CaM/SiNW-FET was also promising tool to study protein—protein or protein—small observed for the association of CaM with a peptide cover- molecule interactions and can be further applied to biomed- ing the IQ domain at the C-terminal of the N-type VGCC in ical diagnosis. the absence of Ca2+ (Fig. 8(i)), which is consistent with a previous study [90]. Finally in Fig. 8(j), two control exper- iments were performed to ensure that CaM/SiNW-FET was DNA hybridization Ca2+ channel-specific and that CaM was essential for the detection of N-type VGCCs. In addition to the protein—protein interactions mentioned Lately, Zheng et al. has also demonstrated a new above, SiNW-FETs were adapted for the detection of DNA methodology based on a frequency (f) domain electrical or RNA. Due to the large amount of negative charges in measurement utilizing a SiNW-FET for protein detection the phosphate backbones of DNA or RNA, SiNW-FETs offer [91]. The power spectral density of voltage from a current- a good candidate for monitoring DNA or RNA hybridiza- biased SiNW-FET shows 1/f-dependence in frequency tions, because the hybridizations cause the accumulation domain for the measurements of antibody-functionalized or depletion of charge carriers in the SiNW-FET, leading to a SiNW-FET devices in buffer solution or in the presence of conductance change. Peptide nucleic acid (PNA), an artifi- protein not specific to the antibody receptor. In the pres- cially synthesized polymer similar to DNA, is commonly used ence of the protein (antigen) which can be recognized in biological research, especially in DNA or RNA hybridiza- specifically by the antibody-functionalized SiNW-FET, the tions. As shown in Fig. 9(a), PNA hybridizes with DNA by frequency spectrum exhibits a Lorentzian shape with a char- base pairing through hydrogen bonds. Because PNA has no acteristic frequency of several kHz. They observed the shape phosphate groups in its backbone, the binding of PNA/DNA of the frequency spectrum to monitor the binding events, or PNA/RNA strands is stronger than that of DNA/DNA or and further to determine the detection limit. With the help DNA/RNA duplexes due to the lacking of electrostatic repul- of this new method in the frequency-domain measurement, sion. Hahm et al. have reported the real-time and label-free the detection sensitivity was claimed to increase by 10-fold. detection of DNA with a PNA-modified SiNW-FET [92]. In
Silicon nanowire field-effect transistor-based biosensors for biomedical diagnosis and cellular recording investigation 145
that study, PNA was anchored to the SiNW surface by the been successfully developed for a rapid and ultrasensitive
strong interaction between avidin and biotin. The successful diagnostic method of detecting Dengus virus.
PNA—DNA duplex formation was demonstrated by the obser- In addition, this promising method allows the detection of
vation of a sizable increase in conductance in the p-type microRNAs (miRNAs) for early cancer diagnosis [94]. MiRNAs
PNA-modified SiNW-FET, because of the negatively charged have been characterized to play a significant role in the cell
phosphate backbones of DNA. Even the surface of the PNA- development and to be related to a number of cancers and
modified SiNW-FET was covered with a layer of avidin; this neurological disorders. Therefore, the detection of miRNAs
ultrasensitive biosensor for sensing DNA is capable of detect- becomes more and more important in the field of medi-
ing down to 10 fM. cal science. As illustrated in Fig. 10(d), a PNA-immobilized
The strategy utilizing PNA as a capture receptor has SiNW-FET was used to probe miRNA by detecting PNA-miRNA
also been applied to study the detection sensitivity of a hybridization via base pairing. As shown in Fig. 10(e), from
SiNW-FET by examining the distance from a binding site the resultant resistance change in the PNA-immobilized
of charged analytes to the SiNW surface [81]. The illustra- SiNW-FET, this approach exhibits an excellent detecting
tion in Fig. 9(b) shows that the distance from the bound specificity capable of discriminating a single base mis-
DNA to the SiNW surface can vary by controlling the loca- match in miRNA. Moreover, as depicted in Fig. 10(f), the
tion of the hybridization site. The neutral character of PNA application of a PNA-functionalized SiNW-FET to probe the
avoids background electric charges to interfere with the hybridization with complementary miRNAs is obviously pref-
binding phenomenon and allows for the hybridization to be erential to a DNA-functionalized SiNW-FET, again indicating
performed in a low ionic-strength environment with a high that neutral PNA prefers to hybridize miRNAs. Also, the
signal-to-noise ratio. The species of target DNAs with dif- PNA-functionalized SiNW-FET sensor is capable of sensing
ferent nucleotides (nt) in the study were designed to be a specific miRNA in total RNA extracted from HeLa cells.
separated into a 22-nt fully complementary, a 19-nt comple- This technique provides a promising tool for early cancer
mentary, a 16-nt complementary, a 13-nt complementary, a detection in which the species and the amount of miRNAs
10-nt complementary, and a 7-nt complementary DNA frag- in the cancer cells were suggested to be different from
ment. In addition, a non-complementary DNA was used as those of normal cells. The combination of PNA and SiNW-FET
a control. The hybridization of PNA—DNA was monitored by provides a powerful technique to detect target molecules
the resistance change that results from the accumulation rapidly and precisely. This PNA-functionalized SiNW-FET sen-
of negative charges on the n-type PNA-modified SiNW-FET. sor could also be applied in medical diagnosis of cancer cell
The resistance changes due to the hybridizations of the PNA growth or other diseases by simply varying the sequences of
receptors with these seven different target DNAs at two the PNA capture receptors.
different concentrations have been recorded in Fig. 9(c).
It is noted that the resistance change of the 7-nt com-
plementary DNA (∼11%) is much smaller than that of fully
complementary DNA (∼50%). The result reveals that when Peptide—small molecule interaction
the complementary segments become shorter, which means
that the distance between the bound DNA and the SiNW sur- The SiNW-FET system has also been applied to study
face becomes longer, the ability of SiNW-FET to detect DNA peptide—small molecule interactions, including ammonia
hybridization is reduced exponentially, as shown in Fig. 9(d). (NH3 ) and acetic acid (AcOH) [95]. As shown in Fig. 11(a),
This observation suggests that the corresponding detection the specific peptides were modified covalently to a SiNW-
sensitivity is mostly dependent on the distance of the charge FET (referred to as peptide/SiNW-FET). X-ray photoelectron
layer to the SiNW surface. spectroscopy and water contact angle were utilized to
verify the successful attachment of peptides on the SiNW-
FET surface (Fig. 11(b)). To test the selectivity of a
peptide/SiNW-FET to AcOH, several experiments have been
conducted to detect the AcOH diluted in acetone, which
Viral infection monitoring and early cancer is a similar molecule to AcOH. In Fig. 11(c), an obvi-
detection ous increase (black curve) is obtained by subtracting the
response caused by the binding of AcOH to peptide/SiNW-
Specific PNA-modified SiNW-FET sensors have recently been FET (referred to as AcOH-peptide/SiNW-FET, green curve)
established to diagnose Dengus virus infection [93]. As rep- from that of the addition of AcOH to peptide-free/SiNW-
resented schematically in Fig. 10(a), the synthetic PNA FET (blue curve), indicating that the peptide/SiNW-FET
receptors were first anchored to the SiNW-FET surface. has an excellent specificity to AcOH in compound chem-
A specific fragment (69 bp) derived from Dengus serotype ical backgrounds. Moreover, the performances of both
2 (DEN-2) virus genome sequences was selected as the AcOH-peptide/SiNW-FET and NH3 -peptide/SiNW-FET were
target DNA and amplified by the reverse transcription- investigated in simulated breath backgrounds as a closer
polymerase chain reaction (RT-PCR). Distinctive resistance approximation toward medical applications. As demon-
changes between the two different PNA receptors (i.e., strated in Fig. 11(d), the electrical response from both FET
complementary and non-complementary to the target DNAs) sensors can be observed after introducing the target analyte
can be distinguished, as seen in Fig. 10(b). The detection (AcOH or NH3 ) in a background of 6% CO2 . These results show
limit of this biosensor based on SiNW-FET was claimed to that peptide/SiNW-FET can be used to monitor the exhaled
be 10 fM (Fig. 10(c)). These investigations suggest that the breath content at high sensitivities and is able to serve as
PNA-modified SiNW-FET sensor incorporated with RT-PCR has an electronic nose for further medical diagnosis.You can also read
