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With the global population prevalence of diabetes surpassing 463 million cases in 2019 and diabetes leading to millions of deaths each year, there is a critical need for feasible, rapid, and non-invasive methodologies for continuous blood glucose monitoring in contrast to the current procedures that are either invasive, complicated, or expensive. Breath analysis is a viable methodology for non-invasive diabetes management owing to its potential for multiple disease diagnoses, the nominal requirement of sample processing, and immense sample accessibility; however, the development of functional commercial sensors is challenging due to the low concentration of volatile organic compounds (VOCs) present in exhaled breath and the confounding factors influencing the exhaled breath profile. Given the complexity of the topic and the skyrocketing spread of diabetes, a multifarious review of exhaled breath analysis for diabetes monitoring is essential to track the technological progress in the field and comprehend the obstacles in developing a breath analysis-based diabetes management system. In this review, we consolidate the relevance of exhaled breath analysis through a critical assessment of current technologies and recent advancements in sensing methods to address the shortcomings associated with blood glucose monitoring. We provide a detailed assessment of the intricacies involved in the development of non-invasive diabetes monitoring devices. In addition, we spotlight the need to consider breath biomarker clusters as opposed to standalone biomarkers for the clinical applicability of exhaled breath monitoring. We present potential VOC clusters suitable for diabetes management and highlight the recent buildout of breath sensing methodologies, focusing on novel sensing materials and transduction mechanisms. Finally, we portray a multifaceted comparison of exhaled breath analysis for diabetes monitoring and highlight remaining challenges on the path to realizing breath analysis as a non-invasive healthcare approach.

A triboelectric nanogenerator (TENG) electrode sensitive to the adsorption of water molecules has been introduced to create a self-powered humidity sensor. Graphene oxide (GO) nanosheets and graphene oxide nanoribbon (GONR) possessing oxygenated functional groups, as well as high dielectric constants, have been proposed as appropriate candidates for this purpose. GO papers have been fabricated in three forms, i.e. pure GO paper, uniform composites of GONR and GO, and double-layer structures of GONR on top of GO. Results showed that all of the prepared paper-based TENGs revealed excellent performances by maximum output voltage above 300 V. As active humidity sensors, the maximum voltage response values of 57%, 124%, and 78% were obtained for GO, GONR+GO, and GONR/GO TENGs, respectively. Besides high sensitivity and precision of all variants, GO+GONR TENG demonstrated a rapid response/recovery behavior (0.3/0.5 s). This phenomenon can be attributed to the higher oxygenated groups and defects on the edges of GONR, which leads to facilitating the bulk diffusion of water molecules. Our results open new avenues of GONR application as an additive to enhance the performance of self-powered humidity sensors, as well as conventional hygrometers.

A grid configuration based on the aligned nickel oxide nanowire (NiO NW) for humidity sensing were fabricated through the oxidation of ferromagnetic nickel NWs prepared by a template-assisted electrodeposition process. Their structure and elemental compositions were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and energy-dispersive X-ray analyses (EDAX). The room-temperature humidity sensing behavior of the NiO NWs was investigated successfully based on DC and AC impedance spectroscopy (IS) method in frequencies range 10 Hz to 2 MHz. The sensors showed excellent humidity sensing characteristics such as a high response of about 66 with rather rapid response-recovery times about 9 and 2 s for 90%RH, and good stability. The equivalent circuits were simulated for impedance responses to humidity in the range of 40–90% RH. According to the results, ionic conduction via NW-NW junctions as well as NW-electrode interfaces in the grid configuration is responsible for sensing behavior.

Triboelectric nanogenerators (TENGs) offer an emerging market of self-sufficient power sources, converting the mechanical energy of the environment to electricity. Recently reported high power densities for the TENGs provide new applications opportunities, such as self-powered sensors. Here in this research, a flexible graphene oxide (GO) paper was fabricated through a straightforward method and utilized as the electrode of TENGs. Outstanding power density as high as 1.3 W.m−2, an open-circuit voltage up to 870 V, and a current density of 1.4 µA.cm−2 has been extracted in vertical contact-separation mode. The all-flexible TENG has been employed as a self-powered humidity sensor to investigate the effect of raising humidity on the output voltage and current by applying mechanical agitation in two forms of using a tapping device and finger tapping. Due to the presence of superficial functional groups on the GO paper, water molecules are inclined to be adsorbed, resulting in a considerable reduction in both generated voltage (from 144 V to 14 V) and current (from 23 µA to 3.7 µA) within the range of relative humidity of 20% to 99%. These results provide a promising applicability of the first suggested sensitive self-powered GO TENG humidity sensor in portable/wearable electronics.

TiS2, as a family member of Transition Metal Dichalcogenides (TMDs), shows a unique property of semi-metallic to semiconductor transition when going from bulk to few layered structure. In this paper, an electromechanical resonator based on TiS2 ribbons with semi-metallic characteristic is presented. Electrical readout of the mechanical vibratory response of TiS2 ribbons is measured by employing electrostatic actuation. A typical resonator includes a number of overlapped ribbons with approximate width and thickness of 600 nm and 250 nm respectively along with a suspended length of 5 μm. A typical resonator shows a resonance frequency of around 153 MHz with a quality factor (Q) of about 1000 in air at ambient conditions. Also, the resonance frequency of the introduced resonator increases as the bias potential is increased due to the electrostatic spring-stiffening effect. Moreover, finite element simulation shows that the reported frequency of resonance and extracted electrical parameters are relevant.

The room temperature polar vapor sensing behavior of a graphene-TiS3 heterojunction material and TiS3 nanoribbons is described. The nanoribbons were synthesized via chemical vapor transport (CVT) and their structure was investigated by scanning electron microscopy, high resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, Raman and Fourier transform infrared spectroscopies. The gas sensing performance was assessed by following the changes in their resistivities. Sensing devices were fabricated with gold contacts and with lithographically patterned graphene (Gr) electrodes in a heterojunction Gr-TiS3-Gr. The gold contacted TiS3 device has a rather linear I-V behavior while the Gr-TiS3-Gr heterojunction forms a contact with a higher Schottky barrier (250 meV). The I-V responses of the sensors were recorded at room temperature at a relative humidity of 55% and for different ethanol vapor concentrations (varying from 2 to 20 ppm). The plots indicate an increase in the resistance of Gr-TiS3-Gr due to adsorption of water and ethanol with a relatively high sensing response (~495% at 2 ppm). The results reveal that stable responses to 2 ppm concentrations of ethanol are achieved at room temperature. The response and recovery times are around 8 s and 72 s, respectively. Weaker responses are obtained for methanol and acetone.

Objective Substantial effort has been put into designing DNA-based biosensors, which are commonly used to detect presence of known sequences including the quantification of gene expression. Porous silicon (PSi), as a nanostructured base, has been commonly used in the fabrication of optimally transducing biosensors. Given that the function of any PSi-based biosensor is highly dependent on its nanomorphology, we systematically optimized a PSi biosensor based on reflectometric interference spectroscopy (RIS) detecting the high penetrance breast cancer susceptibility gene, BRCA1. Materials and Methods In this experimental study, PSi pore sizes on the PSi surface were controlled for optimum filling with DNA oligonucleotides and surface roughness was optimized for obtaining higher resolution RIS patterns. In addition, the influence of two different organic electrolyte mixtures on the formation and morphology of the pores, based on various current densities and etching times on doped p-type silicon, were examined. Moreover, we introduce two cleaning processes which can efficiently remove the undesirable outer parasitic layer created during PSi formation. Results of all the optimization steps were observed by field emission scanning electron microscopy (FE-SEM). Results DNA sensing reached its optimum when PSi was formed in a two-step process in the ethanol electrolyte accompanied by removal of the parasitic layer in NaOH solution. These optimal conditions, which result in pore sizes of approximately 20 nm as well as a low surface roughness, provide a considerable RIS shift upon complementary sequence hybridization, suggesting efficient detectability. Conclusion We demonstrate that the optimal conditions identified here makes PSi an attractive solid-phase DNA-based biosensing method and may be used to not only detect full complementary DNA sequences, but it may also be used for detecting point mutations such as single nucleotide substitutions and indels.

The room temperature polar vapor sensing behavior of a graphene-TiS

Highly sensitive flexible humidity sensors based on graphene quantum dots (GQDs) were developed. The GQDs were prepared using a facile hydrothermal method and characterized considering morphological, structural, and compositional experiments. Then, their humidity sensing properties in correlation with flexibility characteristics were investigated. Good selectivity and response (∼390 for a RH change of 99%), broad detection range (1–100% RH), rather short response and recovery times (12 and 43 s, respectively) as well as flexibility were obtained, demonstrating that the GQD sensors have potential for application in wearable electronics and RH monitoring. Detection of hydrogen (H2) gas by the GQDs confirms the exchange of a proton through adsorbed H+ species in the H2O sensing mechanism. Considering the structure of the sensing material and RT performance, capillary condensation resulting in the formation of a conductive path by water molecules is suggested as the dominant sensing mechanism at high RH.

Titanium trisulfide (TiS3), a transition metal chalcogenide, bears the potential to replace silicon, when taking the form of nanoflakes, due to its favorable band gap and optical response. In this paper, we investigate the response of TiS3 nanoflakes to gas detection through a careful quantum computational approach and a few succinct measurements. The computations are benchmarked and compared with a relevant experiment at each step, where their results/conclusions are discussed. The most stable surface of TiS3 particles is determined as (001), in agreement with the literature. The adsorption of 5 gas molecules is characterized through formulating and estimating their adsorption intensity values, rather than using singled-out values of binding energies. This formulation, which is rooted in a statistical view of the gas adsorption process, distinguishes H2 and CH4 molecules from H2O and O2 explicitly and unambiguously through comparing their adsorption profiles. The difference in the adsorption intensities thus predicts and elucidates the difference in the sensing behaviour of TiS3 particles. This work suggests that the computationally obtained profile for the adsorption spectrum of gas molecules serves as a tool/criterion to predict the selectivity of their detection by TiS3.

A new triple-component sensor for detection of H2 was developed based on porous silicon and CNTs. An increase in deposition of CVD catalysis was shown to promote a high and fast response. Also, it was shown that the composite system exhibited good selectivity.

We report high-efficiency CdTe/CdS core/shell nanocrystals synthesized in water by epitaxially growing CdS shells on aqueous CdTe cores at room temperature, enabled by the controlled release of S species under low-intensity ultraviolet (UV) light illumination. The resulting photo-induced dissociation of S2O32− ions conveniently triggers the formation of critical two-dimensional CdS epitaxy on the CdTe surface at room temperature, as opposed to initiating the growth of individual CdS core-only nanocrystals. This controlled colloidal hetero-epitaxy leads to a substantial increase in the photoluminescence (PL) quantum yield (QY) of the shelled nanocrystals in water (reaching 64%). With a systematic set of studies, the maximum PL QY is found to be almost independent of the illuminating UV intensity, while the shell formation kinetics required for reaching the maximum QY linearly depends on the illuminating UV intensity. A stability study of the QD films in air at various temperatures shows highly improved thermal stability of the shelled QDs (up to 120 °C in ambient air). These results indicate that the proposed aqueous CdTe/CdS core/shell nanocrystals hold great promise for applications requiring efficiency and stability. [Figure not available: see fulltext.] © 2015, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

Pursuing a sensing structure with a large effective surface area, partial ordered arrays of ZnO nanorods with flower-like structures are introduced for gas sensing applications. Room temperature H2S response of the grown structure shows significant enhancement after modification with Au nanoparticles. High response (about 1270 at 6 ppm H2S gas) and selectivity were achieved by depositing an Au layer with nominal thickness ∼6 nm. X-ray photoelectron spectroscopy (XPS) was utilized to describe the H2S sensing mechanism.

Pd doped WO3 fibers were synthesized by electro-spinning. The sol gel method was employed to prepare peroxopolytungstic acid (P-PTA). Palladium chloride and Polyvinyl pyrrolidone (PVP) was dissolved in the sol Pd:WO3 = 10% molar ratio. The prepared sol was loaded into a syringe connected to a high voltage of 18.3 kV and electrospun fibers were collected on the alumina substrates. Scanning electron microscope (SEM), X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques were used to analyze the crystal structure and chemical composition of the fibers after heat treatment at 500 °C. Resistance-sensing measurements exhibited a sensitivity of about 30 at 500 ppm hydrogen in air, and the response and recovery times were about 20 and 30 s, respectively, at 300 °C. Hydrogen gas sensing mechanism of the sensor was also studied.

A novel approach for particles size separation based on dielectrophoresis (DEP) method is introduced and analyzed both computationally and experimentally. The proposed technique has been utilized for the separation of polystyrene (PS) particles with 8, 6 and 2 μm diameters passing through microchannels with planar electrodes. The performance of the technique has been computationally analyzed using the finite element method (FEM). Considering the structure of the planar electrodes, we propose an approach based on applying an electric potential between one of the global electrodes, and a needle touched the top of the cell suspension. Results of both simulation and experiment show that by applying a proper voltage with 500 kHz frequency, the PS particles with 6 and 8 μm diameters will freeze in the direction of the channel while the smaller particles can pass by and exit from the outlet of the channel. Results have shown that the DEP force exerted on the particles by the middle electrode (needle) is stronger and the separation yield is higher compared with the conventional methods.

The sol gel method was employed to prepare peroxopolytungstic acid (P-PTA). Palladium chloride salt was dissolved in the sol with different Pd:W molar ratios and coated on Al2O3 substrates by spin coating method. XRD and XPS techniques were used to analyze the crystal structure and chemical composition of the films before and after heat treatment at 500 °C. We observed that Pd can modify the growth kinetic of tungsten trioxide nanoparticles by reducing the crystallite size and as a result can improve hydrogen sensitivity. Resistance-sensing measurements indicated sensitivity of about 2.5 × 104 at room temperature in hydrogen concentration of 0.1% in air. Considering all sensing parameters, an optimum working temperature of 100 °C was obtained.

In this paper we present flexible strain sensors made of graphene flakes fabricated, characterized, and analyzed for the electrical actuation and readout of their mechanical vibratory response in strain-sensing applications. For a typical suspended graphene membrane fabricated with an approximate length of 10 μm, a mechanical resonance frequency around 136 MHz with a quality factor (Q) of ∼60 in air under ambient conditions was observed. The applied strain can shift the resonance frequency substantially, which is found to be related to the alteration of physical dimension and the built-in strain in the graphene flake. Strain sensing was performed using both planar and nonplanar surfaces (bending with different radii of curvature) as well as by stretching with different elongations. © 2015 IOP Publishing Ltd.

This paper reports on hydrogen sensing based graphene oxide hybrid with Co-based metal organic frameworks (Co-MOFs@GO) prepared by the hydrothermal process. The texture and morphology of the hybrid were characterized by powder x-ray diffraction, scanning electron microscopy and Brunauer-Emmett-Teller analysis. Porous flower like structures assembled from Co-MOFs and GO flakes with sufficient specific surface area are obtained, which are ideal for gas molecules diffusion and interactions. Sensing performance of Co-MOFs@GO were tested and also improved by sputtering platinum (Pt) as a catalyst. The Pt-sputtered Co-MOFs@GO show outstanding hydrogen resistive-sensing with response and recovery times below 12 s at 15 °C. Also, they show stable, repeatable and selective responses to the target gas which make it suitable for the development of a high performance hydrogen sensor.

Solution processed graphene variants including graphene oxide (GO) and reduced graphene oxide (RGO) are promising materials for potential optoelectronic applications. To date, efficiency of the excitation energy transfer into GO and RGO thin layers has not been investigated in terms of donor-acceptor separation distance. In the present work, we study nonradiative energy transfer (NRET) from CdSe/CdS quantum dots into single and/or double layer GO or RGO using time-resolved fluorescence spectroscopy. We observe shorter lifetimes as the separation distance between the QDs and GO or RGO decreases. In accordance with these lifetimes, the rates reveal the presence of two different mechanisms dominating the NRET. Here we show that excitonic NRET is predominant at longer intervals while both excitonic and nonexcitonic NRET exist at shorter distances. In addition, we find the NRET rate behavior to be strongly dependent on the reduction degree of the GO-based layers. We obtain high NRET efficiency levels of ∼97 and ∼89% for the closest separation of the QD-RGO pair and the QD-GO pair, respectively. These results indicate that strong NRET from QDs into thin layer GO and RGO makes these solution-processable thin films promising candidates for light harvesting and detection systems. © 2013 American Chemical Society.

In the present work we report an approach to resistive hydrogen sensing based on graphene quantum dots (GQDs)/WO3 thin films that work reproducibly at low temperatures. GQDs were chemically synthesized and evenly dispersed in WO3 solution with 1:1 molar ratio. The structural evaluation and crystallization of the prepared films was studied by x-ray diffraction, Raman and scanning electron microscopy (SEM) techniques. The SEM images showed uniform distribution of the GQDs in WO3 films with sizes around 50 nm. Raman experiment showed the GQDs are partially reduced with high edge defects as hydroxyl and carboxyl groups which involve both in bridging between WO3 grains via bindings as well as interacting with target gas molecules. GQDs can develop an electron conductive network and shorten the current transport paths inside the sensitive films. As a result, they improved the poor electrical properties and charge transfer of pure WO3. Resistive hydrogen sensing showed significant decrease in the working temperature for GQDs/WO3 films compared to pure WO3 films. The working temperature of about 150 °C with 15 and 40 s response and recovery times are significant characteristics of the introduced sensing structure. Then palladium (Pd) was added as a catalyst in GQDs/WO3 film to make the sensing materials selective to hydrogen. Pd doped film worked at temperature of 120 °C with high selectivity and improved response magnitude to hydrogen gas.

We present fabrication, characterization, and experimental results describing electrical actuation and readout of the mechanical vibratory response of electrospun ZnO nanofibers. For a fiber with an approximate radius of 200 nm and a length of 70 μm, a resonance frequency around 3.62 MHz with a quality factor (Q )o f about 235 in air at ambient conditions is observed. It is shown that the measured frequency of the resonance is consistent with results from finite element simulations. Also, the measurements were performed in an enclosed chamber with controlled levels of ethanol vapor. The adsorption of ethanol causes a shift in the resonance fre- quency of the fibers, which can be related to the concentration of ethanol vapor for sensing purposes.©2014Society

In this paper we present experimental results describing electrical readout of the mechanical vibratory response of graphene-doped fibers by employing electrical actuation. For a fiber resonator with an approximate radius of 850 nm and length of 100 μm, we observed a resonance frequency around 580 kHz with a quality factor (Q) of about 2511 in air at ambient conditions. Through the use of finite element simulations, we show that the reported frequency of resonance is relevant. We also show that the resonance frequency of the fiber resonators decreases as the bias potential is increased due to the electrostatic spring-softening effect.