Review pubs.acs.org/ac
Electrically Transduced Sensors Based on Nanomaterials (2012− 2016) Girija Thesma Chandran,† Xiaowei Li,† Alana Ogata,† and Reginald M. Penner*
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Department of Chemistry, University of California, Irvine, Irvine, California 92697-2025, United States
CONTENTS
Scope of this Review Inorganic Nanomaterials Introduction and Background Nanowires Detection of Vapors and Gases Detection of Dissolved Species in Liquids Inorganic 2D Layers Detection of Vapors and Gases Detection of Dissolved Species in Liquids Metal Nanoparticles Detection of Vapors and Gases Detection of Dissolved Species in Liquids Metal Oxide and Other Inorganic Nanostructures Polymer Nanostructures Introduction and Background Polymer Nanowires Polymer Nanofibers Polymer Nanoparticles Polymer Nanotubes Other Polymer Nanostructures Nanocarbons Introduction and Background Carbon Nanotube and Related Composite Materials Detection of Vapors and Gases Detection of Dissolved Species in Liquids Graphene and Related Composite Materials Detection of Vapors and Gases Detection of Dissolved Species in Liquids Other Nanocarbon Materials Conclusion Author Information Corresponding Author ORCID Author Contributions Notes Biographies Acknowledgments References
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reviews encompassing these other sensing modalities are the following, refs 1−5. The synthetic nanomaterials that have enabled an era of transformative sensor science began to emerge in the mid1980s with the discovery by Kroto, Smalley, and workers6 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 biocompatibility.7−9 Around the same time, Brus and co-workers demonstrated the synthesis of colloidal semiconductor nanocrystals.10 These discoveries of new, zero-dimensional nanomaterials produced tremendous excitement for their sizetunable 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 Iijima11 of the first one-dimensional nanomaterial, single- and multiwalled 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, electrically transduced sensing was launched by this discovery. 2D materials, with tremendous potential for sensors, were discovered later: graphene (200415), 2D transition metal dichalcogenide (TMD) monolayers (201016−19), and MXenes (201120). With few exceptions, sensors exploiting electrical transduction can be classified as chemiresistors, field-effect 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 the conductivity of the channel. Charged analyte species adsorbed or otherwise
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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 © 2016 American Chemical Society
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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 are 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 H2O 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. However, 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 550 K 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 (ΔR/R0) is compared for 1000 ppm of H2 gas for Pt nanowire widths of 40, 25, 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 et al.46 evaluated Pt-modified Pd NWs (Pd@Pt) fabricated with controlled Pt coverage over the range from 0 to 10 Pt MLs. The goal is to impart the more rapid kinetics seen for Pt nanowires39,45 to Pd nanowires while retaining the H2 concentration-dependent resistance response
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 biotinmodified 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 itself. This review is organized according to the composition of the nanomaterial. Three broad classifications are Inorganic Nanomaterials, Polymer Nanostructures, and Nanocarbons. 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.
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INORGANIC NANOMATERIALS Introduction and Background. From a composition perspective, the inorganic nanomaterials category is the most diverse among the three considered here. Included under this umbrella are metal particles23,24 and nanowires,25,26 metal oxide nanowires27 and nanotubes,28 2D transition metal dichalcogenide (TMDC) monolayers,18,29 MXenes (metal carbide and nitride monolayers30,31) 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. Nanowires. Detection of Vapors and Gases. Nanowires (NWs) composed of metals, metal oxides, and semiconductors have played a prominent role 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 nanowires39) or by accepting or donating electrons (for metaloxide nanowires40,41). For semiconducting nanowires configured as 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 the carrier concentration within the nanowire 250
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sensor with a single surface termination is sufficient to distinguish between all 11 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 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 10 μm in length. This electrode detects nitrate by electrochemical reduction to nitrite using a linear potential sweep measurement with a limit-ofdetection of 1.7−3.0 μM, even in solutions contaminated with nitrite and chloride which generate subtractable background currents.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.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 overcoated 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 to 1.0 M. This signal was measurable even 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
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 show 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 to 376 K.46 While H2 sensors often rely on metallic palladium and platinum sensor elements, NW-based 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 n-type SnO2 NWs coated with a p-type Cu2O shell (Cu2O@SnO2). These core−shell NWs are obtained using a combination of vapor transport (forming the SnO2 core) followed by atomic layer deposition (ALD) of the Cu2O shell. Ensembles of hundreds of these nanowires deposited on an interdigitated microwire array were evaluated for the detection of toluene (C7H7) and benzene (C6H6) (both reducing gases), and NO2 (an oxidizing gas) as a function of the dimensions of the Cu2O shell from 0 nm (pristine SnO2) to 80 nm of Cu2O.48 The influence of the Cu2O shell is profound, inverting the direction of the resistance change seen at Cu2O@SnO2 nanowires relative to pure SnO2 nanowires for all three gases: Resistance increases are induced by C7H7 and C6H6 while exposure to NO2 causes a resistance decrease. Performance is also superior for C7H7 and C6H6 as compared with NO2 and this disparity is explained by a model involving the rectification of radial charge flow by the Cu2O−SnO2 n−p junction at the NW surface. NWs composed of the nanocrystalline metal oxides CuO, Cr2O3 and NiO (all p-type) have been applied to the detection of volatile organic compounds (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 to detect VOCs exhibit high sensitivity and rapid recovery times, relative to films of the same metal oxide. For example, ΔR/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 NWs as a means to impart selectivity to a range of VOCs for these FETs. Seven different functional groups are evaluated as receptors for the detection of 11 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 251
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Figure 1. (Left panel) SEM images of (a) fabricated conformal SiNW arrays connected to microcontact regions; (b) magnification of a smooth, trapezoidal single SiNW after the TMAH wet-etching step; (c and d) SiNW arrays after patterning the SiO2/Si3N4 protection layer and magnification of a single nanowire, respectively. (Right panel) Detection response of the fabricated SiNW FET biosensor in different ALCAM concentrations in 10% serum. (a) Typical SiNW FET transfer function curves recorded from 0 to 300 fM, 3 pM, 300 pM, and 30 nM ALCAM; VDS = −2.5 V; measuring time 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 252
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Figure 2. MoS2 2D nanosheet FET gas sensor. (a,b left) Schematic and image of the MoS2 monolayer sensor. (a) Schematic diagram of the MoS2 2D nanosheet FET gas sensor. (b) Optical image of processed devices showing the monolayer MoS2 flakes electrically contacted by multiple Au leads. (a−d right) Response of sensors to triethylamine (TEA) exposure. (a) Change in conductivity of the monolayer MoS2 sensor channel upon exposure to a sequence of 0.002% P0 TEA pulses (black line). The solid red line shows the response to exposure of nitrogen only. The solid green and purple lines show the response of the MoS2 and graphene sensors to water vapor pulses (0.025% P0), respectively. (b) Exposure to TEA pulses of increasing concentration from 0.002% P0 to 0.2% P0. (c) The amplitude of the conductivity change increases with TEA concentration. (d) Change in conductivity of a CVD graphene monolayer (red) and CNT-network sensor (black) upon exposure to a sequence of 0.02% PP0 TEA pulses. Reproduced from Perkins, F.K.; Friedman, A.L.; Cobas, E.; Campbell, P.M.; Jernigan, G.G.; Jonker, B.T. Nano Lett. 2013, 13, 668−673 (ref 74). Copyright 2013 American Chemical Society.
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 Guo 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 (⟨diameter⟩ ≈ 5 nm) AuCu alloy nanowires which were then utilized to construct nonenzymatic H2O2 biosensors. A limit-of-detection for H2O2 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 (Cu3P NWs/CF). In this process, Cu(OH)2 nanowires prepared by electrochemical oxidation of the copper foam are transformed topotactically into Cu3P nanowires by gas phase phosphidation using NaH2PO2 in an argon flow at 300 °C. The Cu3P NWs/CF sensors show a high activity for H2O2 reduction with a detection limit of 2 nM and selectivity in the presence of ascorbic acid, dopamine, glucose, and citric acid. The detection of H2O2 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) Ni2P nanoarrays supported on
conductive carbon cloth (Ni2P NA/CC) as a high-performance catalyst electrode for glucose oxidation in alkaline media. Ni2P 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. Inorganic 2D Layers. The layered transition metal dichalcogenides (TMDs) are among 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 Bi2Te3, 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 (hBN) 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 non-TMD 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 have been employed for the preparation of 2D materials involving mechanical exfoliation, sonication assisted exfoliation, shear exfoliation, and lithium ion intercalation. Reviews discussing the 253
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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 Schottkycontacted 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 of chemiresistors composed of 2D tin disulfide (SnS2) flakes (⟨diameter⟩ ≈ 100 nm) drop-deposited 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 limitof-detection for this sensor is estimated to be 5 ppb showed less response compared to signal responses done for cyclic exposures (Figure 3e,f). HPNT sensors were subjected to mechanical stress, such as imposing a bend radius of 10 mm, and 95% of the original response was maintained after 100 bending cycles.173 In 2016, Xue et al.174 demonstrated a facile synthesis of highly oriented single crystal PPY nanotubes and utilized these nanostructures in an NH3 gas sensor. Single crystalline PPY nanotubes were obtained both by the template-assisted electrochemical deposition and microcold-wall vapor phase deposition using anodic aluminum oxide (AAO) hard templates. The AAO template produced homogeneous single crystal PPY nanotubes with inner and outer diameters of 80 nm and 60 nm, respectively. When configured as chemiresistors, a prompt increased resistance was seen for exposure to NH3 and a limit-of-detection for NH3 of 0.05 ppb was obtained for these materials. A time-invariant steady state resistance was not seen on the time scale of 10 s, but response times were under 5 s followed by recovery times of 30 s.174 Polypyrrole nanotubes additionally acted as excellent transducers in electrolyte-gated FET chemical sensors. Song et al.175 designed a FET nanobioelectronic tongue capable of “tasting” compounds responsible for bitter flavors. The hTAS2R38 gene determines whether humans can taste bitter foods, correlated to pheylthiocarbamid (PTC) and propylthiouracil (PROP) compounds. Two hTAS2R38 strains, taster (PAV) and nontaster (AVI), were expressed from Escherichia coli and covalently attached to PPY nanotubes. The hTAS2R38modified PPY nanotubes were configured as electrolyte gated FETs, operating in phosphate buffer saline. hTAS2R38-FETs sensed PTC and PROP in real time at concentrations ranging from 1 fM to 10 fM. Stability tests showed that 80% of the original sensor capability was maintained after 7 days. Sensors were expanded to PAV and AVI type FETs able to or unable, respectively, to target bitter tastants. PAV-type FET showed selective response to bitter tastants when tested against sweet and umami compounds.175 Hybrid reduced graphene oxide (rGO-PPY) nanotubes incorporated into electrolyte-gated FETs were employed for the detection of H2O2. Entrapment of rGO and PPY nanotubes 264
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Figure 4. A spearhead nano-FET made by deposition of polypyrrole (PPy) to comprise the channel (a) works as a highly sensitive pH biosensor. From I versus VG curves (b) changes in pH value can be measured as change of average source−drain current or source−drain peak currents (c) as well as shift in gate voltage (d). The pH-sensitive PPy nano-FET is applied to measure the local pH in the microenvironment of cell samples (e). Prior to cell measurements the PPy nano-FET is calibrated with physiological pH variations from 5 to 7.5 (f). Local pH measurements by alternating vertical approach and withdrawal of the PPy nano-FET probe to breast cancer tissue (g), a cluster of melanoma cells (h), and a single melanoma cell (i). Reproduced from Zhang, Y. et al. ACS Nano 2016, 10, 3214−3221 (ref 183). Copyright 2016 American Chemical Society.
resistive circuit elements arranged in a chain.180 A series arrangement for nanolayers was associated with a one order of magnitude higher sensitivity than a parallel arrangement in this study. Gan et al.181 prepared HxTiS2 nanosheets by exfoliation and modified these by the solution-phase polymerization of PANI in order to prepare electrochemical sensors for Cu(II). The resulting HTiS2/PANI nanosheets were dispersed onto glassy carbon electrodes and used for the chromoamperometric determination of Cu(II). Relative to eight other metal ion species, Cu(II) was observed to preferentially partition into the porous HTiS2/PANI nanosheet network, providing for significant selectivity for its detection. HTiS2/PANI nanosheet amperometric sensors produced a limit-of-detection of 0.7 nM Cu(II) and a linear range extending from 25 nM to 5 μM. Sensor stability was assessed by storing devices in air for 1 month, and these aged devices showed no change in peak current upon reintroduction of buffer and Cu2+.181 Wang et al.182 fabricated branched polyethylenimine (BPEI) nanobeads and functionalized these with ferrocene by coupling ferrocenecarboxaldehyde (FcCHO) to BPEI to form redox polymer nanobeads, with diameters ranging from 50 to 500 nm. Redox polymer beads were mixed with glucose oxidase (GOD) and PEDOT:PSS on screen-printed carbon electrodes. Operating these electrodes at potentials positive of 0.20 V vs Ag/AgCl enables the amperometric detection of glucose.182 An ultimate glucose limit-of-detection of 66 μM was reported which was improved from sensors where the PEDOT:PSS component was omitted. BPEI/GOx/PEDOT:PSS sensors specifically detected glucose in the linear range of 0.5 mM to
4.5 mM when tested against ascorbic acid, dopamine, and uric acid.182 Zhang et al.183 demonstrated an FET based on a nanospearshaped dual carbon electrode for ATP detection.183 Nanospear carbon electrodes were fabricated by deposition inside doublebarrel nanopipets (Figure 4a). A polypyrrole nanojunction was deposited in between carbon nanoelectrodes to form a 200 nm transistor channel resulting in a pH sensitive FET sensor (Figure 4b−d). Solutions of hexokinase with ATP were implemented to induce a change in proton concentration within biological cells (Figure 4e). PPy nanoFETs produced a calculated limit-of-detection of 10 nM, and these devices demonstrated the ability to function in a pH range from 2.4 to 7.5 (Figure 4f). The spear-shaped nanoFET enabled precise localized detection and probed pH levels in tissue, cell clusters, and single cell surfaces (Figure 4g−i).183
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NANOCARBONS Introduction and Background. Nanocarbons encompasses a diverse range of carbon materials. The most prominent members of this category are carbon nanotubes (CNT), which can be further differentiated into single-walled carbon nanotubes (SWCNTs) in the case where only one monolayer of cylindrical carbons is present and multiwalled carbon nanotubes (MWCNTs) where multiple cylindrical carbon layers are nested one within another,184,185 graphene, graphene oxide (GO), reduced graphene oxide (rGO), spherical fullerenes,186,187 carbon nanofibers,188,189 and carbon nanofoam.190 We discuss the use of these nanocarbons for chemical sensing.191−224 265
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Figure 5. (a−d, Left) (a) SEM image of a representative MWCNT decorated with Ag NCs connecting two gold electrode fingers. Typically a small number of Ag NC−MWCNT hybrids bridge the gold electrode fingers in this study. (b) SEM image of the enlarged view of Ag NC−MWCNT structure as marked in part a. (c) TEM image of Ag NC−MWCNT hybrid structures. The inset is an SAED pattern of the hybrid demonstrating Ag crystallinity. (d) HRTEM image of Ag NCMWCNT hybrid nanostructure. The inset is the enlarged view of Ag structure as marked on the Ag NC. (b,d, Middle) (b) I−V characteristics of Ag NC−MWCNT hybrid sensors in airflow and in 1% NH3 flow. (d) Five sensing cycles of the Ag NC− MWCNT hybrid sensor to 1% NH3, indicating a good stability. (a,b, Right) (a) Dynamic response (ΔR/R0) of the Ag NC−MWCNT hybrid sensor when exposed to different concentrations (C) of NH3. (b) Curve fit of the sensor response (ΔR/R0) as a function of NH3 concentration. The inset is a linear fitting of 1/S (R0/ΔR) vs 1/C. Reproduced from Cui S.; Pu, H.; Lu, G.; Wen, Z.; Mattson, E. C.; Hirschmugl, C.; Gajdardziska-Josifovska, M.; Weinert, M.; Chen, J. ACS Appl. Mater. Interfaces, 2012, 4, 4898−4904 (ref 232). Copyright 2012 American Chemical Society.
Carbon Nanotube and Related Composite Materials. Detection of Vapors and Gases. Dai and co-workers225,226 launched the era of nanocarbon sensing when they described the catalytic growth of carbon SWNTs on Fe(NO 3 ) 3 nanoparticles on silicon in 1998.225 These SWCNTs were much more easily synthesized, and their location on an inert substrate could be predetermined by the positioning of catalyst particles. In 2001, Dai and co-workers226 prepared the first H2 sensors based upon single SWCNTs that had been modified with 5 Å of vapor-deposited palladium. Surprisingly, 40 ppm of H2 in air induced an increase in resistance by a factor of 2, in spite of the fact that a minute amount of Pd had been deposited onto these SWCNTs. Rapid response times of 20−30 s were also reported.226 Further work by Collins and co-workers227 confirmed that Pd NPs that were located at defects in SWNTs were capable of altering the electrical resistance of these structures by 3 orders of magnitude when exposed to H2. For pristine, defect-free SWCNTs, in contrast, Pd nanoclusters randomly distributed along the sidewall, produced just a 2-fold increase in resistance, a result similar to that reported earlier by Dia and his students. These results provided clear evidence that defect sites on SWCNTs sidewalls provided much stronger electronic coupling between immobilized Pd NPs as compared with defect-free SWCNTs and the opportunity to achieve extraordinary signal amplification.227 In work reported in 2011, just prior to the review period, Ganzhorn et al.228 made an important observation concerning the sensitivity of H2 sensors on the chirality of SWCNTs. Using H2 sensors consisting of unmodified SWCNTs, they exploited a sensing mechanism in which a Pd/SWCNT Schottky barrier, present at Pd electrical contacts, was modulated by H2 exposure. The Schottky barrier height can be affected by variation of the Pd work function, which is reduced upon conversion to PdHx. In addition, hydrogen-induced surface dipoles at the Pd/SWCNT interface can also influence the
barrier height. Interestingly, the chirality and diameters of SWCNTs exerted a strong influence on H2 sensing behavior. For example, SWCNTs with a chirality of (9, 7) responded to 100 ppm of H2 gas balanced in air with a 100-fold conductance change, however, comparatively little response was observed by using (6, 5) semiconducting SWCNTs.228 The influence of metal contacts also provided the motivation for a 2014 study by Zhang et al.229 who evaluated the effects caused by different metal contacts (Ag, Pt, Pd, Cr) for H2 detection at SWCNTs. Two sensor categories were distinguished in this work: Category 1 sensors were based on Schottky contacts (either one or two) to SWCNTs that were not decorated with Pd. Their gas sensing response is mainly attributed to the modulation of the barrier height for contacts between the metal semiconductor (MS) at the interface between the electrode and the transducer. Category 2 sensors involved single or multiple SWCNTs that were decorated with Pd. Category 2 sensors outperformed Category 1 sensors in this study both in terms of sensitivity and response rate. These systems produced a wide sensing range of 25 ppm to 2000 ppm with a LODH2 of 25 ppm. Jung et al.230 reported H2 sensors based on carbon nanotubes coated with Pt. The carbon nanotube sheets were well aligned on glass substrates and Pt layers were prepared by e-beam deposition. Compared to nanowire H2 sensors,39,231 Pt@CNT sensors showed faster response to H2 for concentrations of 3− 33 vol % at room temperature.230 Reversible, room temperature NH3 gas sensing can be achieved by employing multiwalled carbon nanotubes functionalized with Ag nanocrystals (AgNC−MWCNTs) (Figure 5).232 This type of NH3 sensor responded to 1 vol % NH3 with the a relative resistance change of 9% at room temperature. The gas sensing selectivity was also evaluated by testing 1 % H2, 100 ppm CO, and 100 ppm NO2, and Ag NC−MWCNTs sensors responded selectively to NH3 in these experiments. 266
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Moon et al.233 were also able to detect H2S using Co3O4 nanoparticle-decorated SWCNT chemiresistors. These sensors demonstrated good selectivity for H2S relative to a series of reducing gases including NH3, H2, and CH4. The detection range was ≈10 ppm to ≈150 ppm, and the sensitivity was strongly enhanced by increasing the operating temperature from 100 to 250 °C, yet overheating the sensor at 300 °C reduced its sensitivity.233 H2S is a broad-spectrum toxic gas that can affect the nervous system while also poisoning other metabolic systems in the human body. Since 10 ppm of H2S is the OSHA permissible exposure limit (PEL), H2S safety sensors having a LOD well below 10 ppm are required. Asad et al.234 evaluated CuOmodified SWCNTs as chemiresistors for the detection of H2S. They observed a rapid ≈7 s response and ≈28 s recovery upon H2S exposure of 1 ppm for devices operating at 150 °C and demonstrated a lowest limit-of-detection of 100 ppb at this temperature.234 Lower temperatures produced significantly lower sensitivity for these systems. These sensors produced a stable signal to 1 ppm of H2S for 30 days.234 Carbon monoxide, CO, is a toxic gas that is capable of binding to hemoglobin. CO toxicity is acute for CO concentrations exceeding ≈35 ppm. As in the case of H2S, sensors capable of detecting CO at much lower concentrations are needed. Choi et al.235 described Au−SWCNTs sensors were able to detect CO in the range of a few ppm. This type of chemiresistive sensors also responded to NH3, C3H5OH, C6H6, and C7H6 by generating a resistance increase, while NO2 induced a resistance decrease for this system. Detection of Dissolved Species in Liquids. Heavy metal ions including Hg2+, Pd2+, and Cd2+ are potent environmental toxins. CNT-based composite materials have shown great promise as compact and highly sensitive sensors for these ions. SWCNTs functionalized with either of two chelating agents for Co2+ were evaluated for the detection of Co2+ in aqueous solutions by Gou et al.236 In this study, the response of polyazomethine polymer (PAM)−SWCNTS composite layers was compared with SWCNT composites with a more rigid, salophen macrocycle (MAC) comprised of identical ion coordinating ligands. Sensitivity, detection range, and limit-ofdetection were significantly better for PAM-modified SWCNT layers, which were characterized by linear signal versus log [Co2+] from 10−12 M to 10−2 M.236 In 2014, Wang et al.237 studied the influence of multiwalled CNTs (MWCNTs) on the detection of metal ions by Nafionmodified glassy carbon electrodes using voltammetric measurements. Using the Eu3+ as a model ionic species, mixtures of Nafion and MWCNTs were deposited on glassy carbon electrodes and the voltammetry of this composite polymermodified electrode was compared with pure Nafion films based upon the measurement of the Eu3+/2+ redox couple. The concentration of MWCNTs within these Nafion films was varied from 0.5 mg/mL to 2 mg/mL. The sensitivity of the sensor for Eu3+ increased by a factor of ≈10 over this range. Nafion−MWCNTs sensors exhibited a dynamic detection range from 1−100 nM Eu3+. CNTs decorated with metal NPs and metal oxide NOs have been widely studied as nonenzymatic glucose sensors in which the nanoparticles selectively catalyzed glucose electrooxidation. Zhao et al.211 developed glucose sensors based on MWCNTs coated with cubic Cu nanoparticles. These electrodes were prepared using an arc discharge method for the production of dense cylindrical MWCNTs layers that were then function-
alized with cubic copper nanocrystals using substrate-enhanced electroless deposition (SEED). The resulting Cu−MWCNTs sensors were able to oxidize glucose in 0.1 M NaOH alkaline solution and produced a sensitivity for glucose oxidation of 922 μA mM−1 cm−2, a dynamic range of 0.5−7.5 mM, and a LODglucose of 2 μM.211 Along similar lines, Tang et al.219 modified CNTs surface with NiCoO2 nanosheets for nonenzymatic glucose detection. Deposition of nanostructured NiCoO2 was induced by employing an intermediate layer of sulfonated polystyrene. This composite functioned as an efficient electrocatalyst for glucose oxidation producing a sensitivity as high as 1424.41 μA cm−1 mM−1, with a LODglucose of 10 μM. A third glucose sensor was described by Zhao et al.220 who electrodeposited Pt and Ni alloy nanoclusters (PtxNi1−x, x = 0.1−0.9) on MWCNTs. In this specific case, an optimum Pt/Ni ratio of 3:7 was determined, providing a sensitivity of 0.94 mA mM−1 cm−2. A LODglucose 0.3 μM (S/N = 3) was possible at a relatively negative oxidation potential of −0.30 V versus Ag/AgCl, which eliminated interference from biomolecules including ascorbic acid, urea, dopamine, galactose, and others.220 A radically different sensor architecture and mechanism of detection for glucose was described by Lee et al.222 who employed SWCNT yarns infiltrated with a hyaluronic hydrogel doped with boronic acid (BA), a molecule with a wellestablished affinity for glucose. The influence of glucose exposure for this system was the generation of torque about the axis of the yarn, caused by the influence of glucose on the swelling of the hydrogel/BA/CNT yarn. The angle-of-rotation of a paddle structure affixed to the yarn was monitored as a function of glucose concentration, and large amplitude reversible rotations of this angle were observed of up to 40°/ mm of yarn. Rotation of the yarn axis increased from 5 to 100 mM of glucose for this unique sensor that required no external power for its operation. Zhang et al.205 used a composite of lanthanum metal, La, with MWCNTs to simultaneously determine ascorbic acid, uric acid, dopamine, and nitrite ions. The MWCNTs were processed by using HNO3 to form carboxyl groups in order to promote nucleation and growth of La nanoparticles, which were prepared by the thermal decomposition of LaN3O66H2O. La-MWCNTs were deposited on glassy carbon and the voltametric response of solutions containing all four analyte species showed discrete oxidation waves for all four species. Amperometric detection was then carried out at potentials ranging from +0.08 V vs Ag/AgCl for copamine to +0.75 V for NO3−. This system afforded a limit-of-detection for dopamine and uric acid of 0.04 μM and a limit-of-detection of 0.40 μM for both ascorbic acid and nitrite ions.205 A similar approach by Tsierkezos et al.194 involved nitrogen (N)-doped MWCNTs composite materials containing metal nanoparticles can electrochemically oxidize organic compounds, including asorbic acid, uric acid, and dopamine.194 These workers evaluated the electrochemical response of glassy carbon electrodes modified with a layer of N-doped MWCNTs decorated with Rh, Pd, Ir, and Pt nanoparticles, all of which had mean diameters in the 2.6−2.7 nm diameter range and gold nanoparticles with a 14 nm diameter. The concentrations of ascorbic acid, dopamine, and uric acid in phosphate buffer were determined using cyclic voltammetry. The greatest sensitivity and lowest LOD for these analyses were obtained for the 14 nm gold nanoparticles and sensitivity decreased in the order: Pt, Ir, Pd, and Rh. 267
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Figure 6. (a,e, Left) Schematic diagram of a Pt_rGO-immobilized radio frequency identification (RFID) sensor tag. (e) Photographs of the RFID sensor tag under different deformations (bending and twisting). (a−d, Right) (a) Normalized resistance changes upon sequential exposure to (b) various concentrations of hydrogen gas, and (c) periodic exposure to 50-ppm hydrogen gas. (d) Calibration curves of Pt_rGOs as a function of hydrogen gas concentration (black, rGO; red, Pt_rGO_0.1; blue, Pt_rGO_1; green, Pt_rGO_10). Reproduced from Lee J. S.; Oh J.; Jun, J.; Jang J. ACS Nano 2015, 9, 7783−7790 (ref 243). Copyright 2015 American Chemical Society.
and the signal amplitude was also increased by ≈30% when the humidity was increased from 10% to 30%.239 Pak et al.240 described the preparation of periodic arrays of graphene nanoribbons (GNRs) with a width of 200 nm width and a 1 μm pitch, produced using laser interference lithography. Pd nanoparticles were then deposited on the GNRs by physical vapor deposition in order to sensitize these GNRs for H2 detection. Pd-decorated GNR arrays exhibited a rectangular sensing curve with response and recovery times of 60 s (at 1000 ppm) and 80% recovery within 90 s in a nitrogen ambient. Configuring these arrays as chemiresistors enables the detection of H2 gas in the range of 30−1000 ppm and showed the signal change was ≈5% for 1000 ppm of H2 exposure.240 Hong et al.241 also developed H2 sensors based on single layer graphene modified with Pd nanoparticles, but in this case Pd deposition was achieved by galvanic displacement of Cu. The sacrificial copper source in this case was the substrate for the graphene, a copper surface. Following Pd deposition, both the graphene and the Pd nanoparticles were lifted off the copper by embedding in a cast poly(methyl methacrylate) (PMMA) layer. This PMMA membrane with its associated graphene layer and Pd nanoparticles was then transferred to gold electrodes. The PMMA layer remained in place and acted as a “pre-filter” on top of the sensing element in order to filter out other analytes including CO, CH4, NO2 that might interfere with H2 detection. The PMMA/Pd NP/SLG hybrid sensor exhibited a reproducible response to 2% H2 with a response time of
