Blood Gas Analyzer Sensor

Functional Nanomaterials

Yingying Jian1, Wenwen Hu2, Zhenhuan Zhao1, Pengfei Cheng2, Hossam Haick1,3 *,

Mingshui Yao4 *, Weiwei Wu1 *

Yingying Jian and Wenwen Hu contributed equally to this review.

1

School of Advanced Materials and Nanotechnology, Interdisciplinary Research Center of Smart Sensors,

Highlights

• This review gives a thinking based on the generic mechanisms rather than simply dividing them as different types of combination of materials, which is unique and valuable for understanding and developing the novel hybrid materials in the future. • The hybrid materials, their sensing mechanism, and their applications are systematically reviewed. Critical thinking and ideas regard- ing the orientation of the development of hybrid material-based gas sensor in the future are also discussed.

ABSTRACT  Chemi-resistive sensors based on hybrid functional mate- rials are promising candidates for gas sensing with high responsivity, good selectivity, fast response/recovery, great stability/repeatability, room-working temperature, low cost, and easy-to-fabricate, for ver- satile applications. This progress report reviews the advantages and advances of these sensing structures compared with the single constitu- ent, according to five main sensing forms: manipulating/constructing heterojunctions, catalytic reaction, charge transfer, charge carrier trans- port, molecular binding/sieving, and their combinations. Promises and challenges of the advances of each form are presented and discussed.

Critical thinking and ideas regarding the orientation of the development of hybrid material-based gas sensor in the future are discussed. KEYWORDS  Gas sensor; Hybrid; Chemi-resistor; Functional

Cite As

Nano-Micro Lett.

1  Introduction

Monitoring and recording chemical stimulus or variations in the environment are increasingly important in future production and daily life of human health . Achieving this goal relies on the availability of high-performance sen- sor units that are capable of detecting gas analytes, such as volatile/semi-volatile organic compounds (VOCs/SVOCs) highly rich regarding critical information for the detection, monitoring and closed-loop control in many fields, including medicine, food industry, environmental monitoring, public security, and agricultural production [2, 3].

An ideal gas sensor requires high responsivity, good selec- tivity, fast response/recovery, great stability/repeatability, room-working temperature, low cost, and easy-to-fabricate for practical applications . To meet those requirements, many types of gas sensors with different transduction forms, e.g., chemi-resistor, field-effect transistor (FET), solid-state electrochemical sensor (SSES), quartz-crystal microbalance (QCM), gas capacitor, surface acoustic wave (SAW), have been well studied and developed. Among them, since the 1960s , a chemi-resistor that contains an active sensing layer bridging a pair of electrodes became a promising can- didate due to its advantages [4–6, 8–12] including easy-to- fabricate, use of very small quantity (milligram level) active materials, wide adoption of sensitive materials, and simple sensing data, which ensure its success in certain commer- cialization opportunities . However, it is rare to find chemi-resistors that can meet these specific requirements.

An emerging approach in chemi-resistors to meet these needs relies on hybrid materials, viz. materials that integrate 2+ single constituents at the nanometer or molecular level , to achieve new and/or enhanced sensing properties.

In this progress report, we review the advances of the hybrid material-based gas sensors concisely and comprehensively. The hybrid materials-based chemi-resistive gas sensors are distinguished, understood, and introduced based on the generic mechanisms rather than simply dividing them as different types of combination of materials. Then, the report, in detail, focuses on the research and development (R&D) aspects of hybrid gas sensors, while presenting and discuss- ing the sensing performances of different types of hybrid materials, and associated enhanced sensing mechanisms.

Promises and challenges toward the future development of each elements are deeply thought and discussed. Critical thinking and ideas regarding the orientation of the develop- ment of hybrid material-based gas sensor in the future are also discussed.

Nanomaterials For Sensing Applications

Chemi-resistors for gas sensing include three main pro- cesses: diffusion/molecule capture unit, surface reaction unit (including charge transfer), and charge carrier trans- port unit (Fig. 1a) . To date, most of these sensors and/ or sensor arrays utilize sensing elements that are based on single material or transduction mechanism, of which intrin- sic sensing activity or additional thermal/photonic energy are usually employed as the driving force to stimulate the sensing effects of target gases (Fig. 1b). The hindrances are unavoidable at several levels: (i) not satisfying long-term stability and sensitivity of organic chemi-resistors due to the high affinity of conductive polymers (CP), such as polyani- line (PANI), polypyrrole (PPy), and polythiophene (PTh), toward volatile organic compounds (VOCs) and humidity existed in the atmosphere; (ii) high operating temperatures (usually > 200 °C), baseline drift, limited selectivity, and oxidation/decomposition of VOCs in the case of inorganic materials (especially metal oxide materials, e.g., ZnO, SnO, ­TiO2, ­SnO2)-based chemi-resistor. A reliable solution for these drawbacks is the design and utilization of new gas sensitive materials based on hybrid inorganic–inorganic , organic–organic , and inorganic–organic materials [8, 9, 12, 16–36].

Using hybrid materials as sensitive transducer offers sev- eral obvious advantages, compared with the single constitu- ent. First, the inexhaustible abundance of hybrid materials (in both the complex constituents and novel nanostructures) makes it possible to involve an almost infinite continuum of variable factors (surface-dependent factor, interface- dependent factor, and structure-dependent factor) to gen-

Erate New Sensing Behaviors (Fig. 1C) . Second,

with hybrid material, more chemical/physical processes with different enhanced mechanisms could be introduced to precisely design, regulate, and enhance the sensing per- formance mainly through catalytic reaction with analyte

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67], molecular binding/sieving , and their combina- tions .

3  Hybrid Chemi‑Resistive Gas Sensors

Hybrid materials can perform improved sensing character- istics via one or a combination from five typical hybridizing forms which are categorized into three sensing-dependent factors (Fig. 2a). The first combination relies on catalysis reactions (normally noble metal catalysts, e.g., Pt ,

Pd , Au , And Ag ) Between Analyte Gas And

decorated catalysts on host semi-conductive materials (cat- egorized as surface-dependent factor). The second relies on a fast charge transfer process, viz carrier withdrawal or donation, electron acceptor or acceptor between guest addi- tives and the host material (e.g., carbon nanotubes (CNTs)), reduced graphene oxide (rGO) (categorized as interface- dependent factor) . The third relies on regulating the charge carrier transport in a conductive/semi-conductive materials (e.g., single-wall carbon nanotube (SWCNT)- metallo-supramolecular polymer (MSP), gold nanoparti- cles (GNPs)-thiols, N,N’-diphenyl perylene tetracarboxylic diimide (PTCDI-Ph)/para-sexiphenyl (p-6P)) upon exposure to gas analytes (categorized as interface- and structure- dependent factor) [66, 80, 81]. The fourth relies on manipu- lation/construction of the heterojunctions such as n–n, p–n, p–p, p–n–p heterogeneous semi-conductive materials (cat- egorized as interface-dependent factor) . The last one relies on semiconductors coated by gas molecular sieving/ binding layers or ligands/complexes for selective gas detec- tion (categorized as surface- and structure-dependent fac- tor) [72, 76, 82]. In the following section, we provide more details on each of these combinations. It should be noted that, according to the understanding of authors on chemi- resistors (1. measuring the resistors directly; 2. measuring the current when the device is applied a constant bias volt- age; 3. measuring the partial voltage on the device in parallel

O−

Fig. 1   Schematic illustration of a three gas-sensing units and b sensing reactions. c The enhanced gas-sensing mechanisms for hybrid chemi- resistive nanomaterials. The upper part of c was modified from reference. Reproduced with permission . Copyright 2009, Elsevier

© The Authors

with a constant resistance when the resistance and device is applied a constant bias voltage), we will review here only resistive-change sensing devices in which the contact of sen- sitive materials and electrodes is ohmic (good linearity of I–V curves under DC bias, Fig. 2b); therefore, we exclude devices in which the I–V curves under DC bias are nonlin- ear despite that they exhibit similar resistance changes (e.g., chemical diodes, proton/ions types).

3.1  Hybrid Gas Sensors Based on Catalytic Effects Catalytic effects of hybrid functional nanomaterials con- tribute to high response, fast speed, and low operating temperature via chemical/electronic sensitization, which is usually accompanied by synergistic effects, complementary behavior, and porous structures [50, 52, 83–86]. In addi- tion, the exposed facets (morphologies) of matrix nanograins (facet-dependent chemical activity) and catalytic additives can greatly enhance the sensing properties of hybrid nano- materials [87, 88]. For example, introduction of Cr dopants to ­WO3 polydedra can not only control the specific exposed facets and activation energy, but bring catalytic effects to the matrix . The combining effects led to improved sensitiv- ity and reduced operating temperature. Further hybridiza- tion with catalytic Au nanocrystals—to form Au/Cr–WO3 hybrids—contributed to high sensitivity, fast speed, and reduced working temperature to acetone and benzene due to Au/Cr co-catalysts-enhanced surface reaction (Fig. 3a, b).

The advantages of co-catalysts can improve even further the hybrid materials. A recent example of this approach is Pd/Sb nanocrystals modification of ­SnO2 that Sb and Pd functioned as anti-humidity and catalytic sites, respectively, which remarkably reduced humidity interference and improved responses toward ­H2 (Fig. 3c) [89, 90].

The catalytic effects of loaded catalysts on host-sensitive materials are associated with the contact between catalyst and gas. A gas diffusion-favoured structure can provide addi- tional exposed surface areas and fast speed via a combination of surface- and structure-dependent factors. The introduction of catalytic Ag NCs via e-beam evaporation and calcina- tion into quasi-1D heterostructures significantly enhances the response and selectivity to ethanol (Fig. 3d) [51, 91].

For even further performance, quasi-1D nanostructures with

V

Fig. 2   a Schematic illustration of five typical forms of hybrid functional nanomaterials for chemi-resistive gas sensing; b typical I–V curves of different sensors under DC bias (from left to right, chemi-resistors, chemical diodes, and proton/ions types)

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both porosity and sensitive nanobuilding blocks, namely mesoporous 1D nanofibers/tubes (meso-NF/NTs), have been reported [41, 92–94]. By introducing sacrificial poly- meric colloids and protein-templated catalysts to the solu- tions, meso- and macro-porous Pt-decorated ­SnO2 NTs have been fabricated by electrospinning and sintering in sequence (Fig. 3e, f) . The combined effects of porous nanostruc- tures, fully depleted sensing areas and uniformly distributed Pt nanocatalysts on ­SnO2 NTs allow a highly selective detec- tion of acetone (R5ppm = 192). Similarly, bimetallic PtM

10 Nm

Fig. 3   a SEM images of ­WO3 octahedron, b 2.5 at.% Cr–WO3 truncated octahedron and 10.0 at.% Cr–WO3 cuboid. Reproduced with permis- sion . Copyright 2015, American Chemical Society. c Humidity dependence of sensor response to 200 ppm hydrogen at 350 °C, using (black) undoped ­SnO2, (red) 0.1 mol % Sb-doped ­SnO2, and (green) 0.1 mol.% Pd-loaded and Sb-doped ­SnO2 nanoparticles. Reproduced with permission . Copyright 2016, American Chemical Society. d TEM images of pure ­SnO2, 5Ag–SnO2, 10Ag–SnO2, and 50Ag–SnO2 NW after heating at 450 °C for 2 h and the corresponding response-recovery curves to ethanol gas. Reproduced with permission . Copyright 2011, American Chemical Society. e, f SEM images of Pt–PS–SnO2 NTs (the inset is a schematic illustration). Reproduced with permission . Copyright 2016, Wiley–VCH. g Pattern recognition by PCA using dataset from sensor arrays of PtM-decorated meso-WO3 NFs evaluating real and simulated (diabetes and halitosis) breath. Reproduced with permission . Copyright 2017, Wiley–VCH. h TEM image of Pd nano- right 2012, American Chemical Society. i Gas response of Pt/BP and pristine BP to various ­H2 concentrations (the inset shows the TEM images of Pt/BP). Reproduced with permission . Copyright 2017, American Chemical Society. (Color figure online)

© The Authors

(M = Pd, Rh, and Ni) catalysts can be introduced to meso- WO3 NFs that are then highly selective detectors of acetone and ­H2S gas . Sensors array combined with pattern rec- ognition methods (so-called e-nose) based on three different PtM-decorated meso-WO3 NFs can accurately detect and discriminate the breath of a simulated biomarker through principal component analysis (PCA, Fig. 3g).

On the basis of intensive works on porous 2D ZnO nano- structures [37, 95, 96], Pd NCs have been deposited on porous 2D ZnO nanoplates (host materials) transformed by ­Zn5(CO3)2(OH)6 nanoplates to form 2D Pd/ZnO hybrid nan- oplates (Fig. 3h); these acquire enhanced sensing properties . Similarly, catalytic Pt can be used to decorate the sur- face of BP, which enables RT detection of ­H2 by Pt/BP at RT (Fig. 3i) . When the structure of the host materials is fur- ther upgraded to 3D hierarchical porous (hp) nanostructures, a good gas diffusion platform is obtained with a large loading area of catalysts. By taking the advantage of the opals/poly- meric beads or mesoporous silica/carbon/polymers templat- ing method, hp-MOX thin films (3D hp nanostructures) with certain additives (catalysts) have been developed .

In this simple approach, hp-SnO2-inverted opal thin films loaded with mono-dispersed Pt catalyst (of uniform size of ~5 nm) were prepared (Fig. 4a) . The improved sens- ing responses of Pt-doped ­SnO2-inverted opal thin films were achieved due to increased porosity, electronic sensi- tization, and synergism (Fig. 4b). Similarly, hp–Pt–WO3 or hp–Cr–WO3-inverted opal thin films have been successfully

Toluene

Fig. 4   a Schema of the one-step preparation of Pt-doped ­SnO2-inverted opal films, and b responses comparison of different sensors as a func- tion of CO concentration at 350 °C (insets are the corresponding HRTEM micrographs). Reproduced with permission . Copyright 2010, American Chemical Society. c Responses comparison of different sensors as a function of working temperature to 74 ppm ­NH3 gas (the inset is the corresponding HRTEM micrograph of Pt-WO3-inverted opal films). Reproduced with permission . Copyright 2011, American Chemi- cal Society. d Synthesis of ordered mesoporous ­WO3/Pt hybrids. e Gas response of ­WO3/Pt-0.5 and ­WO3/Pt-0 to different gases (hydrogen, CO, methane, ethanol, ammonia, acetone, benzene, and toluene) at 100 ppm and 125 °C in 55–60% RH (the inset is an FESEM image of the crystal- line ­WO3/Pt-0.5 viewed from the top surface). Reproduce with permission . Copyright 2018, Wiley–VCH

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prepared (Fig. 4c) . Due to the catalytic activation of N–H bond dissociation and effective gas diffusion within the macro-porous structures, excellent ­NH3 responses were obtained for the hp–Pt–WO3 sensor. As another example, well-controlled self-assembly of block copolymers such as poly(ethylene oxide)-blockpolystyrene (PEO–b–PS) could generate a perfect template with a highly ordered struc- ture. Mesoporous ­WO3/Pt with a highly ordered and porous structure (inset of Fig. 4e) could be obtained by using this template and a 2-step pyrolysis process (Fig. 4d, including first treated in inert atmosphere and finally calcinated in air) . Contributed to high surface areas (112–128 m2 g−1), large pore size (13 nm), and well-dispersed catalytic Pt NCs (~ 4 nm), the ­WO3/Pt-0.5 sensor had the highest response and fastest response-recovery speeds (Fig. 4e). Another category of nanostructures of host materials is multi-shell, yolk–shell, and multi-yolk–shell with hollow nanocham-

Ber . With The Hollow Host Nanochamber ­(Sno2)

loading catalytic Pd through spray pyrolysis (Fig. 5a) , sensitivity and selectivity were enhanced due to the unique hierarchical porous structure and uniformly exposed Pd cata- lysts. The representative works are summarized in Table 1.

As aforementioned, greatly improved sensitivity and speed have been realized by well design on the dispersion of catalysts/co-catalysts and gas diffusion-favored struc- tures of hybrid gas sensors based on catalytic effects. The remaining problems of operating temperature and selectiv- ity might be resolved by further combination with charge

0

0

Fig. 5   a Gas responses of dense ­SnO2 spheres, ­SnO2 yolk–shell spheres, and Pd-loaded ­SnO2 yolk–shell spheres to various analytical gases at

© The Authors

transfer (Sect. 3.2) and molecule sieving layers (Sect. 3.5), respectively. Moreover, the newly developed single atomic metal and/or metal cluster-based catalysts with better cata- lytic effect might bring new understandings and chance in such areas .

Effects

Charge transfer happens between decorations and the host materials (good conductivity), which could vary the conduc- tivity of the hybrid materials. This process improves sen- sitivity to the analysts at low temperature or even at room temperature (RT), accompanying fast response and recovery properties . Discrete and uniform ­SnO2 NCs-decorated

Multi-Walled Cnts (Mwcnts) (Fig. 6A) Gave High

performances (response of ~ 180% to 100 ppm of ­NO2) at RT due to the abundance of active sites and easy electron transfer under the assistance of the well-matched work functions of ­SnO2 and MWCNTs. Liu et al. used rGO instead of the carbon nanotubes. This researcher successfully synthesized ­SnO2 QDs/rGO hybrids by a one-step solvent thermal reaction at 180 °C (oleic acid and oleylamine as cap- ping agents) (Fig. 6b) . Due to co-effects of excellent gas adsorption of QDs, effective charge transfer between ­SnO2–rGO interfaces and the superb transport capability of rGO, the sensor responses in 2 s with fully recovery proper- ties upon exposure to 33 to 50 ppm of ­H2S at RT (Fig. 6c).

Meanwhile, the ­SnO2/rGO-based sensor showed an obvi- ous enhanced response compared with the responses of pure ­SnO2- or rGO-based sensors toward ­H2S at 22 °C, in which the rGO acted as a host transducer material. By combin- ing both advantages, 2D ­MoS2 sheets were hybridized with 2D graphene to form rGO/MoS2 aerogel with large surface areas, porous structure, and high electrical conductivity (Fig. 6d) . Efficient and rapid charge transfer across the interface ensured enhancement and fast detection of ­NO2 than bare rGO or ­MoS2 (Fig. 6e). Ascribing to the high specific surface area of porous ­Cu2O nanowires networks and improved conductivity via effective charge transfer, rGO–Cu2O mesocrystals had much higher sensitivity to ­NO2 at RT, surpassing the performance of stand-alone systems of ­Cu2O and rGO sheets (Fig. 6f) .

CPs could also be applied in charge transfer hybrids by replacing the inorganic components. For instance, graphene was combined with PANI to form a hybrid thin film that had improved, reversible, and stable ­NH3 sensing (Fig. 6g, h).

The fast electron transfer between hybrids and ­NH3, assisted by π–π interactions of PANI and rGO with low electron transfer energy barrier, led to more electrons transfer from PANI to rGO; this effectively improved the responsivity and response time (Fig. 6i) . Up to date, the detecting gases are limited to strong reducing/oxidizing molecules such as ­NO2, ­NH3, and ­H2S, which hinders the widely application of such hybrid gas sensors. The critical points to overcome this shortcoming may be depicted as: (1) the chemical/elec- tronic modification of the reported charge transfer based hybrids to improve the sensitivity and expand the types of detectable gases (i.e., Pt–SnO2/rGO, details in Sect. 3.6); (2) the development on the new candidate of chemi-resistive decorations with desired absorption–desorption process and well-tailored energy level/energy band gap structure, which always shows low thermal activation energy (< 0.5 eV, i.e., electronic conductive metal–organic frameworks (EC- MOFs), for details see Sect. 3.5). The representative works are summarized in Table 2.

3.3  Hybrid Gas Sensors Based on Regulation of Charge

Transport

Different from charge transfer that simply uses high charge transport capability of the highly conductive component, hybrid gas sensors based on regulation of charge carrier Table 1   Representative works based on catalytic effects

Perature

Refs.

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transport can manipulate the sensing properties by changing carrier concentrations, transportation mode, and/or pathways of charge transport.

A simple and effective way of enhancing responsivity relies on controlling the charge transport by tuning carrier concentrations. For example, a PTCDI-Ph/p-6P ultrathin film was fabricated with a thickness of only 6 nm, of which 5 nm was attributed to (p-6p/p type) asymmetric thickness and 1 nm to (PTCDI-Ph/n type) (Fig. 7a) . Electrons in the PTCDI-Ph were deprived by ­NO2, which simultaneously released the restricted hole in p-6p, and thus influenced the transportation of p-6p; this generated a ­NO2 sensing signal at RT (Fig. 7b). Chi et al. thermal-deposited a high-quality crystalline terrace-like TIPS-pentacene film on p-6P that can easily be positively charged (Fig. 7c). The efficient charge transport ability and low original carrier concentration gave superb ­NO2 sensing in terms of both response/recovery speed (Fig. 7d) and responsivity/sensitivity (Fig. 7e) .

Impressively, when the transport direction of charge carri- ers changes from horizontal to vertical in a vertical diode

Pani

Fig. 6   a SEM image of the sensor after assembly of ­SnO2 nanocrystals onto the MWCNTs (inset is the HRTEM image of a MWCNT uniformly coated with ­SnO2 nanocrystals). Reproduced with permission . Copyright 2009, Wiley–VCH. b TEM image of ­SnO2 QDs/rGO hybrids; and c sensor responses to 50 ppm of different gas at RT. Reproduced with permission . Copyright 2016, American Chemical Society. d Enlarged TEM image demonstrating the ­MoS2 coating of the few-layer graphene scaffold. e Sensor response to 0.5 ppm ­NO2 at different micro- heater temperatures, improving response and recovery time. Reproduced with permission . Copyright 2016, Wiley–VCH. f Sensitivities of ­NO2 sensor for the three devices (inset is a schema of the mechanism of ­NO2 sensing of rGO-Cu2O). Reproduced with permission . Copy- right 2012, American Chemical Society. g Schema of the sensing mechanism of rGO-PANI hybrids. h ­NH3 responses of PANI, rGO and their hybrids at different concentrations. i Repeated ­NH3 responses of PANI, rGO and their hybrids at 10 ppm. Reproduced with permission .

© The Authors

(containing top/down electrodes and VOPc/F16CuPc layers, Fig. 8a), the sensor responded remarkably well to 0.5–5 ppm ­NO2 at RT (Fig. 8c), with an acceptable sensing stability (Fig. 8b) and wide linear working region . More inter- estingly, when the voltage bias is raised from 0.2 to 1.5 V, the sensing ability weakened dramatically (Fig. 8d), being ascribed to the transportation change from ohmic to space charge limited current (SCLC) mode (Fig. 8e) , which may give guidance as to how to choose the bias to control the charge transportation for gas sensors to get them to work under the best conditions.

Another strategy to manipulate the properties of sen- sors based on charge transport is by regulating the con- ductive pathways of conductive-insulate hybrid materials via physical cracks, chemical bindings, or the phase of the component. More specifically, this approach relies indeed on regulating the electron hopping barrier, the interspace between conductive materials, phase-change or their combi- nation. Insulating polymers can be combined with sensitive Table 2   Representative works based on charge transfer effects

Work Temperature

Refs.

R (%)

Fig. 7   a Sensor device configuration and molecular structures of the materials. b The relative response of 1 nm PTCDI-Ph/5 nm p-6P film to ­NO2 pulses. The relative response curve is plotted as a function of time as the devices become exposed to different ­NO2 concentrations. Repro- duced with permission . Copyright 2013, Wiley–VCH. c Sensor device configuration and molecular structures of the materials. d The t90,res, t10,rec and relative recovery after 10-min N2 pulse of the responses to different gases. e Responsivity (R) and sensitivity (S) to different gases.

Reproduced with permission . Copyright 2017, Wiley–VCH

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inorganic materials to fabricate highly sensitive and selective chemi-resistors (Fig. 9a). Hybrid thin films as sandwiched

Pmma/Pd/Pmma (Pmma = Poly(Methyl Methacrylate))

were prepared on a flexible substrate using sputtering and spin coating in sequence (Fig. 9a) . Hybrid thin films with nanogaps formed by 25% mechanical stretching have very selectivity and sensitivity in detecting ­H2 against ­O2, ascribed to the selective penetration of ­H2 in PMMA mem- branes and the density reduction of the cracks formed in the trilayer of the hybrid thin films (Fig. 9b). Adoption of similar principles, but with higher effects, relies on films of GNPs coated with monolayers of thiols . These struc- tures were a good solution for VOCs sensing due to the swelling and shrinkage of molecular chains interacting with

0

Fig. 8   a Schema of the cross section of the tube/organic/mesa structure. VOPc nanopyramids (green cones) provide sufficient space for ­NO2 gas molecules to penetrate the structure. The red particles denote ­NO2 gas molecules. b Stability test in ambient atmosphere: sensitivity (left axis) and baseline (right axis) over time. c Response of periodic ­NO2 exposure. The device is operated under 0.05 V bias applied to the tube electrode.

d Schematic diagram of charge transport and response to ­NO2 under ohmic transport (upper panel) and SCLC transport (lower panel) conditions. e Response of organic nanocrystal diode sensor (device 1) at 0.20, 0.50, and 1.50 V bias applied to the tube electrode. Reproduced with permis- sion . Copyright 2018, Wiley–VCH. (Color figure online)

(A)

Fig. 9   a Schematic diagram of the fabrication of nanogap sensors using PMMA/Pd/PMMA trilayer films on a PDMS substrate. b Schematic diagrams indicating the origins of the difference between the ­H2 detection limits of the Pd nanogap and the PMMA/Pd/PMMA hybrid nanogap sensors. Reproduced with permission . Copyright 2014, Elsevier

© The Authors

VOCs (Fig. 10a) . Assembly of the GNP-based chemi- resistors with a wide variety of functional groups creates sensor arrays with different resistances that can be further varied after interacting with VOCs . The transport of electrons, expressed in electrical resistance, can be dually regulated by controlling the interspaces between GNPs after applying strain to the GNPs-based film, which further influ- ence sensitivities (Fig. 10b) . CNTs are also used as the host with the surface coverage of molecular (MSPs) (inset of Fig. 10c) . Such MSPs could create sensory devices with a dosimetric (time- and concentration-integrated) increase in electrical conductivity triggered by electrophilic chemical substances (Fig. 10c).

In summary, for cases where the resistance decreased upon exposure to target gas, the depression of the off current via carrier concentrations reduction, transportation mode changes and/or physical cracks might be the most effective way to realize high sensitivity. Simultaneously, as presented above, the component used for the controlling charge trans- port of host materials can further contribute to improved speed, long-term stability, and excellent selectivity. The rep- resentative works are summarized in Table 3.

3.4  Hybrid Gas Sensors Based On Heterojunctions

Heterojunction is defined as the interface between two dis- similar semiconductors (one is the host, and the other one is the guest) that form a junction (n–n, p–p, p–n) linked with energy band structure due to the alignment of their fermi level. Notably, although the broad definition of

Or

Or

Fig. 10   a Schema of the working principles of gold NPs thiol-coated thiols gas sensor. Reproduced with permission . Copyright 2014, Else- vier. b Schema of a strain regulated gold NPs-based gas sensor. Reproduce with permission  . Copyright 2015, American Chemical Society. c Response of SWCNT-based chemi-resistive sensors after 50-s exposure to different vapors in ­N2 (concentration in ppm in parentheses). The inset is the schema of the working mechanism. Reproduced with permission . Copyright 2015, American Chemical Society

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heterojunctions covers all types of composites forming a junction in the interface, it is not clear enough for the well understanding of the complicated sensing mechanisms of composites sensing materials. Therefore, in this work, the narrow definition of manipulating/constructing heterojunc- tions is used, which excludes cases of catalytic effects, charge transfer, and charge carrier transport. According to the definition, the junction changes the interface potential energy barriers and regulates the transfer and/or injection of electrons and holes in a precise manner when it interacts with gas analytes. For example, n–n heterojunctions made of ­In2O3 hollow spheres (acetone-sensitive host) coated with ­CeO2 nanoparticles (humidity-sensitive guest) were synthe- sized and characterized as a chemi-resistive film. Expos- ing the layer to various gas analytes has shown selective detection of acetone in the presence of water, taking advan- tage of the chemical interaction between ­CeO2, ­In2O3, and water vapor, which greatly reduces the interfering effects of humidity (Fig. 11a) . By modulating interface poten- tial energy barrier between n–n junctions, as in the case of ­Fe2O3/TiO2 tube-like quasi-1D nanostructures synthesized through a multi-step hydrolysis (Ostwald ripening & thermal reduction), the corresponding sensing performance could be greatly improved (Fig. 11b) . Combining modula- tion of electron transfer over the energy barrier at the per- fect ­SnO2/ZnO heterojunction—fabricated by atomic layer deposition—and UV light generated electron–hole pairs, the sensitivity to ­NO2 could be improved using the ­SnO2/ZnO core-sheath nanowires (Fig. 11c) . By introducing nar- row band gap into the junction, such as in the case of ­In2O3 NCs to ZnO, a good response at visible-light conditions to gas analytes (e.g., formaldehyde) at RT (R100 ppm = 419%) was attainable.

The unique morphology (good compatibility with the devices), nanoscale thickness, and high surface area of 2D nanostructures make them promising as the host materials for chemi-resistive gas sensors. Hybrids of ­SnO2 NCs-dec- orated ­MoS2 nanosheet ­(MoS2/SnO2) were synthesized via hydrolysis-pyrolysis processes (Fig. 11d) with air stability . The ­SnO2 NCs not only enhanced the stability of ­MoS2 nanosheets in dry air, but served as strong dopants for ­MoS2, leading to the changes of conduction channels in the ­MoS2 nanosheets (Fig. 11e). For further improve- ments in the sensing performance, introduction of porosity, such as in the case of ­WO3 lamella-based films loaded with mono-dispersed ­SnO2 QDs (~ 4 nm) (Fig. 11f), could reach high levels . Experimental results show that the porous lamellar-structured ­WO3–SnO2 hybrid films could achieve high response to ­NO2 gas, ascribing the effective insertion of QDs into lamella stacks as a strong electronic sensitization.

Compared with n–n heterojunctions, p–n heterojunc- tions provide a stronger manipulation on interface potential energy barriers, build-in electric field and additional cata- lytic effects in some unique cases. For example, exposing p–CuO nanoparticles loaded on CuO–SnO2 p–n nanowires to ­H2S transformed it to highly conductive CuS (Fig. 12a) , resulting in depleted region change (the p–n junction breakup) and second-order effects (the oxidation of ­H2S by absorbed oxygen) after p-CuO was reversibly gener- ated and removed on the ­SnO2 surface. Without generating Table 3   Representative works based on regulation of charge transport

Work Temperature

Refs.

© The Authors

new chemical compounds, simply tuning the thickness of in situ oxidation layer, rich Te–Te or ­TeO2/TeO2 bridging point contacts and additional p–n heterojunctions (Te/SnO2) contributed to further excellent sensing performances (to CO and ­NO2) of the brush-like heterostructures (Fig. 12b) . The nanorods of p-type coating layer can be replaced by continuous layer to form core-sheath hybrids processing radial modulation of potential energy barriers, for instance, both n-ZnO/p-CoPc (cobalt phthalocyanines, Fig. 12c) and n-SnO2/p-Cu2O (Fig. 12d) . Core-sheath NRs have better sensitivity of the target gases.

Conductive polymers (CPs, e.g., PPy, poly(3,4-ethylenedi- oxythiophene) (PEDOT), PANI) that are p-type components of diverse types of p-n heterojunctions can work at RT or low operating temperature with different working principles.

First, PPy-ZnSnO4, p–n hybrid nanoparticles, can enhance the ­NH3 sensing performance (3–4 times higher) compared with pure PPy and ­ZnSnO4 (Fig. 13a) . The concen- tration of ­NH3 can be quantitatively detected (Fig. 13b) with shorter time of response (26 s) and recovery (24 s) (Fig. 13c). The overall improved performance has been ascribed to the p–n heterojunction, in which the holes at high concentration in PPy and the electrons in ­Zn2SnO4 diffuse into each other to form a built-in electric field of a depletion layer (Fig. 13d). Interaction between ­Zn2SnO4–PPy and ­NH3 broadens the depletion layer, which determines the response, and the speed of response/recovery. When the p–n junction was reinforced as dual p–n junctions (p–n–p) in the hybrids of the hollow ­In2O3 nanofibers (NFs) and PANI (Fig. 13e), the performances were further enhanced (Fig. 13f) .

For CP-based chemi-resistive heterojunctions, unsatisfied sensitivity (response) and long-term stability will be two challenging issues for researchers.

A

Fig. 11   a TEM images and dynamic sensing transients of Ce–In2O3 hollow spheres exposed to 0.5–20 ppm of acetone at 450 °C in dry (red) and RH 80% (blue). Reproduced with permission . Copyright 2016, Wiley–VCH. b TEM image of 1D ­Fe2O3/TiO2 tube-like nanostruc- tures. Reproduced with permission . Copyright 2012, American Chemical Society. c Low-magnification TEM image of ­SnO2/ZnO core– shell nanowires (the inset is the corresponding energy band diagram of the ­SnO2/ZnO system with/without UV light). Reproduced with per- mission . Copyright 2013, American Chemical Society. d High-resolution SEM image showing that ­SnO2 NCs decorate on the ­MoS2 nanosheets (the inset is the corresponding SAED patterns), and e dynamic sensing response of the ­MoS2/SnO2 nanohybrids to different ­NO2 concentrations (the inset is the enlarged sensing response curve for 0.5 ppm ­NO2). Reproduced with permission . Copyright 2012, Wiley– VCH. f Schematic of the sensing mechanism of ­WO3–SnO2 nanoplates and the corresponding responses comparison toward ­NO2 gas. Repro- duced with permission . Copyright 2014, American Chemical Society. (Color figure online)

1 3

Obvious advantages of heterojunction-type chemi-resis- tive hybrids-based gas sensor can be summarized as: (1) higher sensitivity due to manipulations of the potential energy barrier formed by band bending of different com- ponents (e.g., ­Fe2O3/TiO2 tube-like quasi-1D nanostruc-

Tures (N–N) , N-Zno/P-Copc , And N-Sno2/P-

Cu2O core-sheath NRs); (2) improved selectivity to some gases (e.g., CuO–SnO2 p–n nanowires to ­H2S ); (3) promising, although limited so far, to anti-interference gas (e.g., ­CeO2–In2O3 hollow spheres with anti-humidity properties ); (4) avoiding UV-introduced ozone and performance degradation (for example, when narrow-band guest material hybrids with semi-conductive host materi-

Als, E.G., Cds-Zno [123, 124], Zno-Cds , Cdse-Zno

, the room operation temperature can be achieved by the visible-light-driven gas sensing). The representative works are summarized in Table 4.

In summary, the critical points to achieve better perfor- mance in heterojunction-based gas sensor are depicted as: (1) maximum effective contact areas of the interfaces via surface and structure design; (2) matched band structure to facilitate the manipulation of potential energy barrier; (3) additional capability of catalysis of guest additives to host materials; (4) visible-light-driven photocatalytic abilities and good charge carriers separations for light-driven n–n or n–p hybrids; (5) selectivity-improvement-purposed heterojunction design based on specific interaction (e.g., Pd–H2, CuO–H2S, PPy–NH3) or n–p response-type rever- sion ­(Co3O4–SnO2 p–n junctions for ­H2) .

Probing And Sieving Effect

Functionalization, coating or doping in/on the sensing materials, i.e., introduction of the sensing probe, was dem- onstrated as an effective way of improving the selectivity and specificity through a one-lock one-key binding or struc- ture similarity-based combination. Both the inorganic and organic probes have been well developed. The sieving of interferon, especially the humidity in the environment, is an alternative way of improving specificity.

O−

Fig. 12   a SEM image of a single hybrid NW and the schematic illustration of gas-sensing mechanism of CuO–SnO2 p–n nanowires. Repro- duced with permission . Copyright 2013, Elsevier. b SEM images of the as-grown Te-coated ­SnO2 brush-like products prepared at source temperatures of 560 °C. Reproduced with permission . Copyright 2014, American Chemical Society. c The dynamic response-recovery curves for ZnO and n-ZnO/p-CoPc to 10 ppm of ­H2S (the inset is the TEM image of n-ZnO/p-CoPc). Reproduced with permission . Copy- right 2015, American Chemical Society. d TEM image of 1D ­SnO2/CuO nanostructures (left insets are responses to reducing and oxidizing gas as a function of shell thickness, respectively; right insets are the corresponding sensing mechanism). Reproduced with permission . Copy-

8000

Fig. 13   a ­NH3 response to PPy-Zn2SnO4, PPy, and ­Zn2SnO4. b Curve of concentration versus response. c Curve of a single response and recov- ery to ­NH3. d Schema of sensing mechanism. Reproduced with permission . Copyright 2018, Elsevier. e Schema of sensing mechanism. f Gas response of PANI, solid ­In2O3/PANI and hollow ­In2O3/PANI. Reproduced with permission . Copyright 2016, Springer Table 4   Representative works based on heterojunctions

Work Temperature

Refs.

A 3D Sulfonated Rgo Hydrogel (S-Rgoh)-Based Gas

sensor combining chemical functionalization and porous structures was synthesized in a one-step hydrothermal reaction (Fig. 14a) . Addition of a ­NaHSO3 probe dra- matically enhanced the response (Fig. 14c) of ­NO2 with fast recovery (Fig. 14b), assisted by the porous structures of the graphene host. Self-assembled monolayers (SAMs) with suitable alignment of the gas–SAM frontier molecu- lar orbitals (Fig. 14e) with respect to the SAM–NW Fermi level (Fig. 14f); this led to high selectivity and sensitivity to analyte gas . ­SnO2 NWs were modified with amine- terminated SAM and applied as light-driven chemi-resistors working at RT, achieving good ­NO2 sensing performance, the schematic mechanism of which can be found in Fig. 14d.

This concept was extended to porous MOX nanostruc- tures for further enhancements of their sensing properties.

Aptes-Modified Porous ­Wo3 Nanotubes (P-Wo3 Nts

tivity (Fig. 14a), which can be ascribed to the large sur- face area and high gas diffusion rate provided by P-WO3, and selective reaction between ­NO2 and surface SAM with APTES (Fig. 15b, c). The existence of SAM on the surface of inorganic materials (except 2D nanomaterials) limits the working temperature, which greatly weakens the sensing performance, although it could be resolved by UV irradia- tion. Using conductive polymer as the host material with surface SAM functionalization by the “1-stone 2 birds” strategy was promising and novel (Fig. 15d, e) . Superb sensing performances were achieved by combining RT sen- sitivity of CP and good selectivity of SAM (Fig. 15f) .

Figure 16a shows the low cross-sensitivity to humid- ity and other interferon gases by refreshing the regenera- tive surface involving the interaction between facile redox fair ­(Tb3+/Tb4+) and surface OH group (or water vapor) on ­SnO2. This 5 Tb–SnO2-based chemi-resistor achieved high response to acetone exposure . The oleic acid SAM also was effective in screening the effect of humidity (< 350 ppm) when it was layered on PANI surface (Fig. 16b, c) . Although it is not enough for practical application, this demonstration is still valuable in pointing to a promising way to eliminate the interferon of humidity.

Density Of States (Arb. Units)

Fig. 14   a Schema of the synthesis of 3D chemically modified graphene hydrogel. b Dynamic response of the 3D S-RGOH sensor versus time after exposure to 0.2–2 ppm ­NO2. c Plots of the quantitative responses of the S-RGOH and RGOH sensors versus ­NO2 concentration. Repro- duced with permission . Copyright 2017, Wiley–VCH. d, e Schematic illustration of the ­NO2 sensing mechanism by en-APTAS 1-modified are set at 0 eV. For comparison, the DOS of the en-APTAS 1-modified ­SnO2 NW without an adsorbed gas molecule (dashed red line) is shown in each graph. Reproduced with permission . Copyright 2014, Wiley–VCH

© The Authors

Recently, MOF materials are great opportunity in gen- erating sub-nanometer or nanometer pores with high uni- formity. Neat MOFs chemi-resistors were prepared based on hydrophobic MOF (ZIF-67), which showed selective response to VOCs, with slightly interfering effects of NRs powders were synthesized by hydrothermal reaction using a self-template strategy (Fig. 17b) . The chemi- hybrids had satisfactory sensitivity and response time to 100 ppm formaldehyde, even under interfering humidity (Fig. 17c). Mixing the CP with molecularly imprinted polymer (MIP) molecular (Fig. 17d) was another effec- tive approach to improve not only the responsivity, but

Selectivity (Fig.  17F) . Responses Of Interferon

(2,4,6-TNT) and analyte (2,4-DNT) were suppressed and enhanced, respectively, although they are of very similar molecular structure and functional group . However, the speed of response and recovery showed no obvious change (Fig. 17e) . Instead of using functional MOFs as filter film coating on sensing materials to provide addi- tional selectivity and/or sensitization, EC-MOFs are novel emerging materials with regular porosity and conductivity, which are promising for chemi-resistors with high sen- sitivity and selectivity [76, 133–158]. Unlike MOX and MOX-MOFs, which still need additional thermal or pho- tonic energy as the trigger source to activate the sens- ing reaction, EC-MOFs can be directly used as sensitive materials based on their regular micro-porosity, selective frameworks, high electronic conductivity, and RT activity [139, 144–146, 159–166]. Therefore, EC-MOFs are prom- ising components for hybrid gas sensors and will be pow- erful competitors for the new generation of gas sensors.

As mentioned above, the introduction of molecular prob- ing and sieving effect can effectively overcome the poor selectivity problem of chemi-resistors. Up to date, only a few organic/inorganic probe or porous materials with molecule sieving effects have been applied to chemi-resistors to real- ize simple guest–matrix interaction (e.g., -NH2 group with ­NO2, –NO2 group with ­NH3, ­NaHSO3 with ­NO2) or mol- ecule rejection (e.g., anti-humidity, gas molecules with large kinetic diameter). Such cases are ultra-small fraction of the state-of-art gas molecule adsorption and separation areas.

More guest–matrix interactions (e.g., van der Waals inter- actions, hydrogen bond, π–π interactions, weak acid–base interactions) and gas separation design (e.g., channel traffic

Δr/R (%)

Fig. 15   a Selectivity comparison of different gas sensors at the corresponding operating temperature. Schematic illustration of the ­NO2 sens- 2018, Royal Society of Chemistry. d The schema of silane-coated PEDOT:PSS sensor. e The molecular chain of different silane-coated on PEDOT:PSS. f Response to VOCs. Reproduced with permission . Copyright 2018, Royal Society of Chemistry

1 3

effects, framework flexibility), that have been well studied on rGO, polymers (e.g., metal-induced ordered microporous polymers (MMPs), covalent-organic frameworks (COFs), conjugated mesoporous/microporous polymers (CMPs)), MOFs, can be introduced to chemi-resistors for advanced sensing performances. The representative works are sum- marized in Table 5.

Mechanisms

In many cases, multi-forms working on the hybrid materi- als can simultaneously and dramatically improve sensi- tivity and responsivity. When the heterojunction barrier (the ­SnO2/rGO heterostructure interface) was combined

I−I0/I0 (%)

Fig. 16   a Polar plots of the gas responses of pure ­SnO2 and 5 Tb–SnO2 sensors exposed to different gases (at 20 ppm) in dry (yellow) and RH 80% (red) atmospheres, and EDS elemental mapping of Sn, Tb, and O in 5 Tb–SnO2. Reproduced with permission . Copyright 2018, American Chemical Society. b, c Response curves to 1-hexanal at different humidity in bending and flat states. Reproduced with permission . Copyright 2018, Wiley–VCH. (Color figure online)

© The Authors

with catalytic Pt, sensitivity of the ­SnO2/rGO hybrids

To ­H2 Was Greatly Enhanced (Fig. 18A–E) . Hydro-

gen ranging from 0.5 to 3% in air could be quantitatively detected at near RT with response and recovery times of 3–7 and 2–6 s, respectively. Furthermore, when the cata- lytic effect was co-working with the p-n heterojunctions and porous structure (Fig. 19a), ­Co3O4–PdO loaded on n-SnO2 hybrid hollow nanocubes (Fig. 19b) reached selec- tivity and response amplitudes for the detection of acetone superior to those MOF-derived metal oxide sensing lay- ers previously reported. Accordingly, the sensor arrays

N-Sno2 Hncs) Can Clearly Distinguish 1  Ppm Humid

acetone molecules among the seven interfering analytes (Fig. 19c). The reason is electron migration from n-SnO2 to PdO or p-Co3O4 in the multi-junctions significantly influencing the electron depletion regions, which leads to the superb sensitivity (Fig. 19d, e).

D2

Fig. 17   a Effect of environmental humidity on sensitivity of ZIF-67 sensor (inset is the SOD-type structure of ZIF-67). Reproduce with per- template; and c gas response and response time for 100 ppm formaldehyde as a function of the relative humidity. Reproduced with permission . Copyright 2016, American Chemical Society. d Schema of the interaction of 2,4-DNT with the hybrid ingredients (PVA, PPy, and MIP).

e Response of a fabricated chemi-resistor sensor coated with PVA/PPy/MIP hybrids with respect to the 2,4- DNT explosive vapor. f Column curves of PVA/PPy/MIP hybrids to different organic compounds. Reproduced with permission . Copyright 2018, Wiley–VCH Table 5   Representative works based on molecular probing and sieving effect

Work Temperature

Refs.

1 3

When the catalytic effect co-worked with the molecu- lar sieving effect, improved selectivity (anti-interferon) with enhanced sensitivity can be achieved by coating a layer of hydrophobic and thermally catalytic bimetallic MOFs nanowire arrays (NWAs) (Fig. 20a) . The bime- tallic ZIF-CoZn MOF sheathes gave good thermal stability (ZIF-8(Zn)) and excellent thermal catalytic ability on ZnO (ZIF-67(Co)), as well as hydrophobic channels. By com- showed greatly enhanced performance on selectivity (good anti-humidity, Fig. 20b) and also on its response, response and recovery behavior and working temperature (Fig. 20c).

More complicated hybrid nanostructures containing MOX, plasmonic/catalytic NMs, and hydrophobic MOFs, i.e., hybrids had enhanced selective adsorption, detection and oxidation of HCHO and prevented interference from gases such as ­H2O and toluene (Fig. 20f), where Au NRs helped to generate charge carriers on a ZnO surface under visible- light irradiation. The representative works are summarized in Table 6.

4.1  Summary Of Hybrid Gas‑Sensitive Materials

The current progress report reviews advances and the advan- tages of the chemi-resistive hybrid nanomaterials compared with the single constituent, according to five main sensing mechanisms: manipulating/constructing heterojunctions, catalytic reaction, charge transfer, charge transport, molec- ular binding/sieving, and their combinations. Table 7 lists typical chemi-resistive materials for hybrid gas sensors cat- egorized by types of materials and conductivity.

4.2  Applications of Chemi‑Resistive Sensor‑Based

E‑Nose

As the first commercial gas sensor, metal oxide-based chemi-resistors still occupy a leading role in both fundamen- tal researches and commercial devices. Various commercial chemi-resistive gas sensors based on single or hybrid mate- rials have been developed for the detection toward target gases (toxic, flammable, VOCs, explosive, ­H2, etc.) ranging from sub-ppm to saturated vapor, which are widely used in fields including environment monitoring, medical care,

Time (S)

Fig. 18   a TEM and b HRTEM images of Pt-SnO2/rGO. Power spectrum of c the whole HRTEM image and d the region of the HRTEM image indicated by the red circle. e Hydrogen-sensing performance at 50 °C of the materials under investigation: variation in the resistance of the rGO/ SnO2 (solid line) and rGO/SnO2/Pt sensors (dashed line) with pulses of hydrogen at 0.5, 1, 2, and 3%. Reproduced with permission . Copy-

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