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Review
Electrically Transduced Sensors Based on Nanomaterials (2012 – 2016) Girija Thesma Chandran, Xiaowei Li, Alana F Ogata, and Reginald M. Penner Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04687 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016
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Electrically Transduced Sensors Based on Nanomaterials (2012 – 2016) Girija Thesma Chandran# , Xiaowei Li# , Alana Ogata# , and Reginald M. Penner* Department of Chemistry, University of California, Irvine Irvine, CA, 92697-2025 # These
authors contributed equally to this work. Abstract
Chemical sensors and biosensors that exploit sensing elements composed of nanomaterials are reviewed. Attention is focused on sensors for which the transduction of signal by the sensor is electrical, not optical, magnetic, mechanical, etc. Papers relating to fundamental science that is foundational to our understanding of these devices are also discussed. We confine this review to publications appearing during the period from 2012 to 2016. The driver for new analytical science in the area of sensing has been the emergence of new nanomaterials coupled with the development of new methods for preparing nanomaterials. Prominent trends include the rise to prominence of graphene and its derivatives as transducers, the refinement and diversification of nanostructured metal oxide sensors, the impact of electrospinning on the development of polymer nanowire and nanofiber-based sensor architectures, and the first application of new inorganic 2D nanomaterials in sensors including 2D metal carbides and nitrides (MXenes), 2D layered transition metal dichalcogenides (TMDs), and black phosphorus 2D layers.
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Contents Page A Scope of this Review
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B Inorganic Nanomaterials B.1 Introduction and Background . . . . . . . . . . . . B.2 Nanowires . . . . . . . . . . . . . . . . . . . . . . . B.2.1 Detection of Vapors & Gases . . . . . . . . . B.2.2 Detection of Dissolved Species in Liquids . B.3 Inorganic 2D Layers . . . . . . . . . . . . . . . . . . B.3.1 Detection of Vapors & Gases . . . . . . . . . B.3.2 Detection of Dissolved Species in Liquids . B.4 Metal Nanoparticles . . . . . . . . . . . . . . . . . . B.4.1 Detection of Vapors & Gases . . . . . . . . . B.4.2 Detection of Dissolved Species in Liquids . B.5 Metal Oxide and Other Inorganic Nanostructures C Polymer Nanostructures C.1 Introduction and Background . C.2 Polymer Nanowires . . . . . . . C.3 Polymer Nanofibers . . . . . . . C.4 Polymer Nanoparticles . . . . . C.5 Polymer Nanotubes . . . . . . . C.6 Other Polymer Nanostructures
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D Nanocarbons D.1 Introduction and Background . . . . . . . . . . . . D.2 Carbon Nanotube & Related Composite Materials D.2.1 Detection of Vapors & Gases . . . . . . . . . D.2.2 Detection of Dissolved Species in Liquids . D.3 Graphene & Related Composite Materials . . . . . D.3.1 Detection of Vapors & Gases . . . . . . . . . D.3.2 Detection of Dissolved Species in Liquids . D.4 Other Nanocarbon Materials . . . . . . . . . . . . .
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E Conclusion
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F Author Biographies
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G Figures
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H Literature References
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A.
Scope of this Review
In this review we highlight advances in chemical and biological sensors that are based upon nanowires, nanotubes, nanoparticles, and other types of nanostructures described in papers published from 2012 to 2016. This review is confined to sensors for which the transduction of signal by the sensor is electrical, not optical, magnetic, mechanical, etc. Broader reviews encompassing these other sensing modalities are the following: 1–5 The synthetic nanomaterials that have enabled an era of transformative sensor science began to emerge in the mid-1980’s with the discovery by Kroto, Smalley, and workers 6 of buckminster fullerene - highly stable clusters of 60 carbon atoms (C60). The electrochemical, optical, and mechanical properties of fullerenes have since been characterized showing excellent conductivity, high electroactive surface areas, and biocompatability. 7–9 Around the same time, Brus and coworkers demonstrated the synthesis of colloidal semiconductor nanocrystals. 10 These discoveries of new, zero-dimensional nanomaterials produced tremendous excitement for their size-tunable optical properties, but this work did not promptly spark new applications in electrically based sensors because the techniques for immobilizing these nanoparticles and for functionalizing their surfaces did not yet exist. Electrically conductive support materials that could serve as “wires", enabling electrical connections to nanoparticles, were still in the future. These became available in 1991 with the discovery by Sumio Iijima 11 of the first one-dimensional nanomaterial, singleand multi-walled carbon nanotubes (CNTs). Techniques for producing CNTs in quantity using chemical vapor deposition were quickly discovered and refined and relatively rapidly, CNTs were widely available. 12–14 A new era of nanomaterials-based, electricallytransduced sensing was launched by this discovery. 2D materials, with tremendous potential for sensors, were discovered later: graphene (2004 15 ), 2D transition metal dichalcogenide (TMD) monolayers (2010 16–19 ), and MXenes (2011 20 ). Sensors exploiting electrical transduction can be classified as chemiresistors, fieldeffect transistors (FETs), or electrochemical sensors. Chemiresistors are simply electrical conduits having a resistance that is altered by the direct interaction of an analyte molecule with the sensor surface. A well known example are palladium (Pd) nanowire sensors for hydrogen gas. 21 In this case, H2 chemisorbs at the Pd surface forming 2Hads . Subsequent diffusion of H into the Pd sensing element causes the formation of bulk PdHx having a higher electrical resistance than Pd metal. FETs, on the other hand, measure the charge carrier mobility through a channel that is subjected to an applied electric field from a conductive gate electrode. The gate potential causes an accumulation or a depletion of mobile charge carriers, modifying and the conductivity of the channel. Charged analyte species adsorbed or otherwise bound at the channel surface can also gate charge carriers within the channel, leading to measurable changes in the transfer characteristics for the FET. In the first example of FET sensors derived from semiconductor nanowires, Lieber and coworkers demonstrated the detection of streptavidin by biotin-modified silicon nanowire FETs in 2001. 22 Electrochemical sensors derive signal from the electron transfer reaction of an intermediate that may be tethered to the sensor surface, or current associated with the direct or indirect electrooxidation or reduction of the target species 4 ACS Paragon Plus Environment
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itself. This review is organized according to the composition of the nanomaterial. Three broad classifications are: Inorganic Nanomaterials (section B), Polymer Nanostructures (section C), and nanocarbons (Section D). Within each of these three categories, subcategories distinguish between particles, nanowires, 2D layers, and so forth. The ordering of publications within each of these subcategories is chronological by publication date. Finally, the length of this review and the number of permitted literature references is strictly limited. This means that many excellent papers have not been reviewed and cited. We apologize to those authors whose work we have not reviewed.
B. Inorganic Nanomaterials B.1. Introduction and Background
From a composition perspective, the inorganic nanomaterials category is the most diverse amongst the three considered here. Included under this umbrella are metal particles 23,24 and nanowires, 25,26 metal oxide nanowires 27 and nanotubes, 28 2D transition metal dichalcogenide (TMDC) monolayers, 18,29 MXenes (metal carbide and nitride monolayers 30,31 ), and semiconductor nanocrystals, 32 nanowires, 33,34 nanoribbons, 35 and so on. The volume of sensor work in this category is daunting 27,36–38 and we have been very selective in our description. Because of the large volume of work, we have also separated applications relating to the detection of vapors and gases from those involving the detection of liquid-phase species in this section.
B.2. Nanowires B.2.1. Detection of Vapors & Gases Nanowires (NWs) composed of metals, metal oxides, and semiconductors have played a prominent roll in sensing. NWs possess a tremendous surface area-to-volume ratio that is proportional to (radius)−1 . NWs for incorporation into sensors are synthesized using either of two approaches: Top-down fabrication applies standard microfabrication methods to pair away a semiconducting epi-layer to form a nanowire. In this case, nanowires can be directly incorporated into an electrical circuit that is fabricated at the same time as the NW. Bottom-up fabrication, in contrast, involves the synthesis of free-standing nanowires using solution-phase or vapor-phase synthetic methods, and the subsequent positioning of a NW, or ensemble of NWs, on a surface followed by the deposition of electrical contacts. NWs may be configured either as chemisresistor devices using two contacts to each end of the NW(s) or as FETs in which a third gate electrode is employed to apply a transverse electric field. In the chemiresistor mode, the chemisorption of molecules at the NW surface can influence its electronic conductivity either by modulating the surface scattering of charge carriers (for metal nanowires 39 ) or by accepting or donating electrons (at metal oxide nanowires 40,41 ). For semiconducting nanowires configured as field-effect transistors (FETs), a change in the carrier concentration within the nanowire is induced by the capture or adsorption of charged target molecules at the surface of the nanowire, leading to a change in the nanowire resistance. 25,26,33,37,38 The applied gate potential allows 5 ACS Paragon Plus Environment
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the carrier concentration within the nanowire to be tuned, enabling optimization of the sensing response. The sensitivity of nanowire FETs is inversely related to the nanowire diameter. 42,43 Gas sensors are important components in safety control systems that require rapid sensor response and high sensitivity - both of these attributes of NW-based chemiresistor systems. Metal (e.g. palladium) and metal oxide nanowires (e.g SnO2 ) 26,40,44 have been primarily used for this purpose. Palladium nanowire chemiresistors have been used as hydrogen gas (H2 ) sensors since 2002. 21 The transduction of H2 by Pd nanowires involves its dissociative chemisorption on the Pd surface to form Pd-H, diffusion of this adsorbed H into the bulk of the nanowire and the formation of electrically resistive PdHx which results in an increase in the resistance of the Pd NW sensing element. Pt NWs can also dissociate H2 , but no stable bulk hydride exists for Pt, so the transduction mechanism operating for Pd NWs does not apply for Pt. In spite of this fact, in 2012 Yang et al 39 demonstrated that a single Pt NW operating in air exhibits a rapid, reversible decrease in its resistance upon exposure to H2 - exactly the opposite response seen for Pd NWs in air. The transduction mechanism in this case involves a reduction in the diffuse electron scattering of electrons at the Pt surface. In air, a high resistance state caused by efficient diffuse scattering of electrons, exists for the Pt surface which is covered with chemisorbed OH and physisorbed H2 O and O2 . Exposure to H2 produces a lower resistance state - less efficient diffuse scattering of electrons - resulting from the formation of Pt-H and the displacement of oxygen-containing species from the nanowire surface. 39 Because exposure to H2 results in the formation of a Pt-H monolayer, the resistance decrease of a Pt NW saturates at the same resistance value independent of the H2 concentration - a clear liability of this sensing approach. But at a constant gas flow rate, the time rate of change of the NW resistance is correlated with concentration, enabling its determination. Pt NWs exhibit a limit-of-detection of 10 ppm for H2 which is 3 orders of magnitude lower than for Pd NWs of the same size. In addition, Pt nanowires are much faster. For example, Pt nanowires operating at 550K show response times that are 1/100th those of Pd nanowires for the detection of [H2 ]≈1%. 39 Platinum NW arrays employing the same physics have been used for the detection of H2 by Yoo et al 45 who fabricated periodic arrays of 40 nm width Pt NW arrays at 500 nm pitch over a large, 5 × 5 mm2 area. Nanowire fabrication entails the use of a polystyrene (PS) etch mask, coating of this mask with an evaporated Pt film, and reactive Ar+ ion etching to remove PS thereby producing linear nanowires. 45 The size dependence of the H2 sensor signal (∆R/R0 ) is compared for 1000 ppm of H2 gas for Pt nanowire widths of 40 nm, 25 nm, and 10 nm and the signal is observed to increase in amplitude from 0.7% to 5.2% over this size range, qualitatively as expected for the electron scattering mechanism operating in this system. 45 The disadvantage of the surface-scattering mechanism, already clear from Yang’s work in 2012, 39 is the insensitivity of the resistance change to the concentration of H2 caused by the fact that signal saturation coincides with saturation of the Pt surface with hydride. 39,45 This problem is mitigated by employing a Pt monolayer (ML) on a Pd nanowire. Li 6 ACS Paragon Plus Environment
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et al 46 evaluated Pt-modified Pd NWs (Pd@Pt) fabricated with controlled Pt coverage over the range from 0-10 Pt MLs. The goal is to impart the more rapid kinetics seen for Pt nanowires 39,45 to Pd nanowires while retaining the H2 concentration-dependent resistance response that is characteristic of the Pd nanowires. In pursuit of this goal, an electrodeposition method for preparing Pd nanowires coated with a single monolayer (ML) of Pt was devised. 46 The resulting Pd@Pt NWs shows a prompt and reversible increase in resistance upon exposure to H2 in air, caused by the conversion of Pd to more resistive PdHx . The addition of 1.0 monolayer(ML) of Pt to the Pd surface alters the H2 detection properties of Pd@Pt NWs in two ways: First, the amplitude of the relative resistance change measured at each H2 concentration is slightly reduced, and second, response and recovery rates are both accelerated across temperatures from 294◦ K to 376◦ K. 46 While H2 sensors often rely on metallic palladium and platinum sensor elements, NWbased gas sensing is dominated by metal-oxide NWs. 36,44 Metal-oxide NWs have two advantages in comparison with their traditional thin and thick film counterparts: First, operating temperatures and power consumption are both reduced relative to film based devices, and second, integration of the sensor with microelectronics is facilitated for both FET and chemiresistor type devices. 47 While solid metal oxide nanowires have already been extensively investigated, 36,44 an interesting variant described by Kim et al 48 are ntype SnO2 NWs coated with a p-type Cu2 O shell (Cu2 O@SnO2 ). These core-shell NWs are obtained using a combination of vapor transport (forming the SnO2 core) followed by atomic layer deposition (ALD) deposition of the Cu2 O shell. Ensembles of hundreds of these nanowires deposited on an interdigitated microwire array were evaluated for the detection of toluene (C7 H7 ) and benzene (C6 H6 ) (both reducing gases), and NO2 (an oxidizing gas) as a function of the dimensions of the Cu2 O shell from 0 nm (pristine SnO2 ) to 80 nm of Cu2 O. 48 The influence of the Cu2O shell is profound, inverting the direction of the resistance change seen at Cu2 O@SnO2 nanowires relative to pure SnO2 nanowires for all three gases: Resistance increases are induced by C7 H7 and C6 H6 while exposure to NO2 causes a resistance decrease. Performance is also superior for C7 H7 and C6 H6 as compared with NO2 and this disparity is explained by a model involving the rectification of radial charge flow by the Cu2 O – SnO2 n–p junction at the NW surface. NWs composed of the nanocrystalline metal oxides CuO, Cr2 O3 and NiO (all p-type) have been applied to the detection of VOCs by Cho et al. 49 In this application, p-type metal oxides confer a number of advantages relative to n-type materials - stability in air, for example, while the absolute sensitivity of the p-type sensor is somewhat lower than for n-type materials in general. Nanowire arrays of these oxides are obtained simply by patterning evaporated layers of Cu, Cr and Ni and then calcining at 450◦ C. Nanowire ensembles configured as chemiresistors detect VOCs exhibit high sensitivity and rapid recovery times, relative to films of the same metal oxide. For example, ∆R/Ra = 30 is observed for at 1 ppm hexane using NiO NWs with recovery from exposure in 30 s. Response times are somewhat slower, several minutes on average for these systems. Recent efforts involving silicon NW FETs have included systems for the detection of gases. Wang et al 50 have conjugated organic functional groups to the surface of silicon 7 ACS Paragon Plus Environment
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NWs as a means to impart selectivity to a range of volatile organic compounds (OCs) for these FETs. Seven different functional groups are evaluated as receptors for the detection of eleven VOCs target species and an artificial neural network (ANN) is employed to parse the sensing data. A surprising conclusion is that a single Si NW sensor with a single surface termination is sufficient to distinguish between all eleven VOCs when the influence of multiple parameters - hole mobility, threshold voltage, subthreshold slope and the source-drain current - are evaluated by ANN for each gas. 50 Shehada et al 51 developed molecularly modified SiNW FETs and demonstrated their use in the detection and classification of many disease “breathprints” (lung cancer, gastric cancer, asthma, and chronic obstructive pulmonary disease). The fabricated SiNW FETs were characterized and optimized based on a training set that correlated their sensitivity and selectivity toward VOCs linked with the various disease breathprints and the best sensors obtained in the training set were then examined under real world clinical conditions using breath samples from 374 subjects. 51 The analysis of the clinical samples showed that the optimized SiNW FETs can detect and discriminate between almost all binary comparisons of the diseases under examination with >80% accuracy demonstrating the potential of this approach to support the diagnosis of many diseases in a direct, nonintrusive way. 51 B.2.2. Detection of Dissolved Species in Liquids In terms of liquid phase sensing, a simple approach involves the measurement of the electrochemical response of nanowire-modified electrodes as a means for detecting dissolved analytes. In one recent example, Stortini et al 52 fabricated ensembles of copper NW electrodes (CuWNEEs) via electrodeposition into track-etched polycarbonate membranes. Dissolution of the polycarbonate with acetone then exposes a “shag carpet” of 108 cm−2 copper nanowires, 400 nm in diameter and 10µm in length. This electrode detects nitrate by electrochemical reduction to nitrite using a linear potential sweep measurement with a limit-of-detection of 1.7–3.0 µM, even in solutions contaminated with nitrite and chloride which generate background currents that can be subtracted. 52 Cao et al 53 have described the application of SiNW FETs for the measurement of ion concentrations in aqueous solution. Their approach involves overlaying an ion-selective membrane (ISM) containing ionophores capable of selectively binding K+ and Na+ over a single Si nanowire which is both back-gated and subjected to a front or electrolyte gate as well. The efficacy for detecting these two ions was assessed in this study which concluded that both ions can be selectively detected in solutions containing both ions over a concentration range from 10−4 to 10−1 M without interference. 53 A similar strategy was assessed by Wipf et al 54 who attached functionalized crown ether ionophores through a pendent thiol moiety directly to a Si NW, modified with a thin (20 nm) thermal SiO2 passivation layer and then over-coated with a thin (20 nm) gold layer. This gold layer facilitates the attachment of a high density of thiol-modified receptors - a dithiol-modified 15-crown-5 in the case of Na+ . A shift in the threshold voltage, Vth , of 44 mV/decade is observed, enabling a detection range of 10−3 M - 1.0 M. This signal was measurable even 8 ACS Paragon Plus Environment
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in solutions of varying pH, potassium (K+ ), and chloride (Cl− ) ions, by making a differential measurement between the functionalized NW and a NW with a bare gold surface (control) illustrating a critical advantage of gold compared to oxide nanowire surfaces: it makes possible a differential measurement against a suitable control electrode. Muratova et al 55 have studied unmodified gold nanowires for the detection of a broad range of species capable of chemisorbing onto gold, including Cl− and dopamine, a neurotransmitter. As in the case of H2 sensing at Pt nanowires discussed above, 39,45 the transduction mechanism involves adsorbate-induced changes in the diffuse surface scattering of electrons within the gold nanowire. A resistance increase is observed for both of the species studied here, and the observed changes in NW resistance made possible the quantification of Cl− over a concentration range of 10−5 M to 10−3 M and dopamine over a concentration range of 10−8 M to 10−5 M. 55 SiNW FETs, first described by Lieber and coworkers in 2001, 22 have emerged as powerful biosensors that produce a direct electrical readout of biomolecule concentration and which are capable of the label-free detection of proteins and nuclei acids. 34 The sensing mechanism for SiNW FET relies on a change in surface charge density after hybridization of the target with a surface-bound receptor. 34 The range of capabilities demonstrated for Si FETs over the past four years is remarkable: Gao et al 56 optimized a SiNW-FET nanosensor for rapid and reliable detection of target DNA with a detection limit of 0.1 fM and high specificity for single-nucleotide polymorphism discrimination. Shen et al 57 demonstrated the selective detection of influenza A viruses down to a limit-of-detection of 29 viruses/µL in clinical exhaled breath condensate samples (diluted by 100-fold) with an analysis time of minutes using SiNW sensor devices. The selectivity of virus detection was demonstrated using H1N1 viruses, 8 iso PGF 2a, and inert nanoparticles. The SiNW sensor device was shown to be reliably applied to the diagnosis of flu in a clinical setting with 2 orders of magnitude less time compared to the standard method RT-qPCR when calibrated by virus standards and EBC controls. 57 Gao et al 58 described a novel approach for rapid, label-free and specific DNA detection by applying rolling circle amplification (RCA) based on SiNW FET. In this case, the RCA process appends additional DNA to the capture probe and the bound target DNA formed by hybridization. The length of added DNA, and the resulting signal produced by the Si FET, increases as a function of time after the RCA process is initiated for a period of up to 15 min. 58 Amplification of signal by a factor of up to three was demonstrated using this unique strategy, making possible a signal-to-noise ratio >20 for 1 fM DNA detection, implying a limit of detection of 50 aM. 58 Tran et al 59 described an integrated translational biosensing technology based on arrays of SiNW FETs that have been preclinically validated for the ultrasensitive detection of the cancer biomarker ALCAM, a ≈100 kDa transmembrane glycoprotein and cancer prognostic marker(Figure 1). A detection limit of 15.5 pg/mL was demonstrated in an analysis requiring less than 30 min and having a dynamic range of five order of magnitude. 59 Few diseases can be reliably diagnosed based upon the measurement of a single disease marker. This fact has motivated efforts to develop multi-target detection systems 9 ACS Paragon Plus Environment
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based upon nanostructures. Lu et al 60 has described a two-channel PDMS microfluidic integrated CMOS-compatible SiNW FET arrays for the label-free and simultaneous detection of cancer biomarkers. The integrated nanowire arrays showed not only sensitivity of cytokeratin 19 fragment (CYFRA21-1) and prostate specific antigen (PSA) with detection to at least 1 fg/mL in buffer solution but also a high degree of selectivity in the presence of other similar cancer biomarkers. In addition, this method was also used to detect both CYFRA21-1 and PSA undiluted biological fluids at concentrations as low as 10 fg/mL. 60 These integrated SiNW-FET devices open the door to opportunities for rapid, point-ofcare (POC) disease testing and the early diagnosis of cancer and other complex diseases. Nanowires can also be used to impart higher sensitivity, and enhanced selectivity, to an electrochemical measurement of an electroactive analyte species. In all of this work, the goal is the direct electrochemical detection of the analyte, eliminating enzymatic amplification of the detection process. We have already made reference to the detection of 52 Also in this vein is recent work by Shim et al 61 NO− 3 using copper nanowire electrodes. who prepared ultra-high surface area electrodes by growing, via vapor transport, a carpet of single crystalline IrO2 NWs (dia. ≈ 30-50 nm) on a micron-scale Pt wire (dia. = 25 µm). Using this nanostructured electrode in a chronoamperometric oxidation mode, H2 O2 and NADH were detected with a limit-of-detection of 5 µM. Yue et al 62 fabricated vertically aligned ZnO NW arrays on 3D graphene foam (ZnO NWA/GF) that were then used to selectively detect uric acid (UA), dopamine (DA), and ascorbic acid (AA) using differential pulse voltammetry. It was shown that thermal annealing of the ZnO was necessary to manifest selectivity on the ZnO surface among UA, DA, and AA. The optimized ZnO NWA/GF electrode was capsble of a a detection limit of 1 nM for UA and DA. 62 This method was further used to detect UA levels in the serum of patients with Parkinson’s disease (PD). The UA level was found to be 25% lower in PD patients than in healthy individuals suggesting that a depressed UA concentration is diagnostic of PD. Electrochemical detection with nanowire electrodes was also explored by Gao et al 63 who prepared (Ti@TiO2 ) core@shell NW electrodes elaborated by the addition of CdS and NiOOH particles (Ti@TiO2/CdS/Ni electrode), using a hybrid hydrothermal and electrodeposition method. This complex system was evaluated for the nonenzymatic detection of glucose using electrochemical oxidation mediated, in principle, by the NiOOH nanoparticles. Under optimized conditions, this sensor displayed a sensitivity for glucose as high as 1100 µA mM−1 cm−2 , a linear range of 0.005 mM to 12 mM, and a limit-ofdetection of 0.35 µM. 63 Wang et al 64 described a wet chemical route for the preparation of diminutive ( ≈ 5 nm) AuCu alloy nanowires which were then utilized to construct nonenzymatic H2 O2 biosensors. A limit-of-detection for H2 O2 of ≈2 nM was demonstrated. 64 Li et al 65 employed a topotactic conversion method to fabricate ensembles of copper(I) phosphide NWs on three-dimensional porous copper foam (Cu3 P NWs/CF). In this process, Cu(OH)2 nanowires prepared by electrochemical oxidation of the copper foam are transformed topotactically into Cu3 P nanowires by gas phase phosphidation using NaH2 PO2 in an argon flow at 300◦ C. The Cu3 P NWs/CF sensors show a high activity for H2 O2 reduction with a detection limit of 2 nM and selectivity in the presence of asco10 ACS Paragon Plus Environment
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bic acid, dopamine, glucose, and citric acid. The detection of H2 O2 released from living mouse leukemic monocyte macrophage cells was also accomplished in this study, highlighting the ability of these sensors to function as a probe for cancerous cells. 65 Chen et al 66 described the use of three-dimensional (3D) Ni2 P nanoarray supported on conductive carbon cloth (Ni2 P NA/CC) as a high-performance catalyst electrode for glucose oxidation in alkaline media. Ni2 P NA/CC as a nonenzymatic glucose sensor, is superior in sensing performances with a short response time of 5 s, a wide detection range of 1 µM to 3 mM, a limit-of-detection of 0.18 µM (S/N = 3), and a response sensitivity of 7800 µA mM−1 cm−2 with satisfactory selectivity and reproducibility.
B.3. Inorganic 2D Layers
The layered transition metal dichalcogenides (TMDs) are amongst the newest nanomaterials to find application in chemical sensors. The first methods for preparing single molecular layers of the TMDs, including MoS2 , WS2 , MoSe2 , MoTe2 , TaSe2 , NbSe2 , NiTe2 , BN, and Bi2 Te3 , were described in 2011. 67 The utility of 2D TMD nanosheets for chemical sensing were immediately apparent and the entirety of the exploration of these new materials has occurred since 2012, during the period covered by this review. The most investigated 2D TMD material is MoS2 which has a direct bandgap of 1.82 eV (one sheet, 1L), and an indirect bandgap of 1.65 eV (2L), 1.35 eV (3L) whereas bulk MoS2 (L = ∞) has an indirect band-gap of 1.2 eV. 68 Hexagonal boron nitride (h-BN) is a non-TMD 2D material that is analogous in structure to graphene but having a band-gap, Eg ∼6 eV, that has so far limited its utility in electrical sensor applications. Black phosphorus is another a non-TMD 2D layered material with Eg ∼2 eV (d) that is already showing promise for chemical sensing applications. FETs based upon single layer MoS2 show current on/off ratios exceeding 108 at room temperature; much higher than that of graphene transistors. 2D layered nanomaterials can also be easily fabricated as chemiresistive FETs that have low power consumption. 19 Since 2001, diverse methods and have been employed for the preparation of 2D materials involving mechanical exfoliation, sonication assisted exfoliation, shear exfoliation, and lithium ion intercalation. Reviews discussing the synthesis, properties, and diverse applications of 2D layered materials are the following. 17–19,29,69,70 B.3.1. Detection of Vapors & Gases Gas sensors based on 2D layered nanomaterials, including 2D TMDs, operate on a charge transfer mechanism. When the sensors are exposed to a reactive gas, adsorption of the gas species onto the surfaces of the sensing channel is associated with the inductive donation or withdrawal of charge, leading a change in the resistance of the sensing element. Upon exposure to air or an inert gas, molecules of the target gas desorb and the sensor resistance returns to the background value. As with nanowires, state-of-the-art gas sensors are based either on chemiresistor or FET architectures. 17,29 In 2012 Li et al 71 reported single and multilayer (L = 1-4) MoS2 films based FETs for NO sensing. The MoS2 films were deposited onto Si/SiO2 substrates using the scotch tape based mechanical exfoliation technique. Although the single layer MoS2 FET showed a rapid and dramatic 11 ACS Paragon Plus Environment
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response upon exposure to NO, its current was found to be unstable while, the 2L, 3L, and 4L MoS2 FET devices exhibited both stable and sensitive responses down to a detection limit of 0.8 ppm NO. 71 A fully solution processed, flexible thin film transistor (TFT) array based on the MoS2 thin film channel and reduced graphene oxide (rGO) electrodes was fabricated by He et al 72 where the TFT array is used as high performance, easy operable and robust gas sensor for the NO2 detection. Functionalization of the MoS2 thin film channel with Pt nanoparticles (PtNPs) further increased the sensitivity by ∼3 times leading to a detection limit of 2 ppb. The sensitivity of the MoS2 TFT decreased dramatically as the thickness of the MoS2 film was increased from 2.0 nm to 18 nm, an effect attributed to its reduced surface area-to-volume ratio ? In 2013, Late et al 73 fabricated MoS2 transistors with different thicknesses ranging from 1L - 5L and assessed these for the detection of NO2 , NH3 , and humidity as a function of gate bias and light irradiation. The results indicated that, compared with L = 1, transistors of two or several MoS2 layers exhibited better sensitivity, more rapid recovery, and amplification of the sensor response using the gate bias and upon exposure to green light. 73 Perkins et al 74 evaluated monolayer MoS2 chemiresistors prepared from monolayer flakes dispersed on SiO2 /Si wafers for the detection of the vapors or VOCs including triethylamine (TEA), tetrahydrofuran (THF), acetone, methanol, dichlorobenzene, nitrotoluene, and dicholropentane(Figure 2). This paper also makes comparisons with graphene and carbon nanotube (CNT)-based FET sensors. Amongst this series of molecules, particularly sensitive detection of TEA, a strong electron donor, is observed leading to an increase in conductivity of the MoS2 sensing element. TEA was thereby detected with a limit-of-detection of 10 ppm. Decreased conductivity, in contrast, was observed upon exposure of analogous CNT and graphene devices to triethylamine. Weaker donors such as THF and acetone produced a smaller or imperceptible change in the conductivity of the MoS2 sheet. Electron accepting species including nitrotoluene produced no response at MoS2 while a strong increase in conductivity was observed at CNT chemiresistors. 74 The disparate responses seen at CNT and graphene versus MoS2 chemiresistors in this study are expected based upon the fact that MoS2 is weakly n-doped while CNTs and graphene are both p-doped. 74 Liu et al 75 in 2014 reported the use of a Schottky-contacted chemical vapor deposition grown monolayer MoS2 as a room temperature chemical sensor. As in prior studies, sensor signal moves in opposite directions for these two gases: NO2 is an oxidizer which withdraws electron density from the MoS2 channel and the conduction band of this material, necessitating the application of a larger positive gate voltage, Vg , to accumulate charge density. NH3 is capable of donating charge via its lone pairs producing the opposite effect on Vg . At a constant Vg in the accumulation regime, NO2 reduces the source-drain current, IDS , while NH3 increases it. The Schottky-contacted MoS2 transistors showed current changes by 2–3 orders of magnitude upon exposure to very low concentrations of NO2 and NH3 yielding a limit-of-detection of 20 ppb and 1 ppm, respectively. Enhanced sensitivity was attributed to modulation of the Schottky barrier height caused by analyte molecule adsorption. 75 Acceleration of the sensor response/recovery speed is seen for gas sensors described by Ou et al 41 consisting 12 ACS Paragon Plus Environment
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of chemiresistors composed of 2D tin disulfide (SnS2 ) flakes ( ≈ 100 nm) dropdeposited on interdigitated Pt electrodes and operating at, or just below, 160◦ C. Sensor fabrication using this approach is dramatically simplified relative to earlier devices in which a single TMD 2D crystallite was used as the channel of an FET. The NO2 limit-ofdetection for this sensor is estimated to be
