This article is an overview of the present and ongoing developments in the field of nanomaterial-based sensors for enabling fast, relatively inexpensive and minimally (or non-) invasive diagnostics of health conditions with follow-up by detecting volatile organic compounds (VOCs) excreted from one or combination of human body fluids and tissues (e.g., blood, urine, breath, skin). Part of the review provides a didactic examination of the concepts and approaches related to emerging sensing materials and transduction techniques linked with the VOC-based non-invasive medical evaluations. We also present and discuss diverse characteristics of these innovative sensors, such as their mode of operation, sensitivity, selectivity and response time, as well as the major approaches proposed for enhancing their ability as hybrid sensors to afford multidimensional sensing and information-based sensing. The other parts of the review give an updated compilation of the past and currently available VOC-based sensors for disease diagnostics. This compilation summarizes all VOCs identified in relation to sickness and sampling origin that links these data with advanced nanomaterial-based sensing technologies. Both strength and pitfalls are discussed and criticized, particularly from the perspective of the information and communication era. Further ideas regarding improvement of sensors, sensor arrays, sensing devices and the proposed workflow are also included.

Synergy between nanomaterials and volatile organic compounds for non-invasive medical evaluation

An ultrahigh sensitive hydrogen sulfide gas sensor was developed utilizing micromachined nanocrystalline SnO 2 –Ag on ceramic wafer (alumina). A polymeric sol–gel process has been successfully developed in fabricating thin film SnO 2 –Ag nanocomposite, which shows excellent sensing characteristics upon exposure to H 2 S as low as 1 ppm at working temperature as low as 70 °C. This sensor also shows higher sensitivity to H 2 S under various humidity conditions, in comparing to commercialized H 2 S sensors as well as published data. The environmental influences on sensor's sensitivity and selectivity have been tested and discussed. The results show that the sensor has a good selectivity due to its less sensitive to common interference gases like Cl 2 , HCl, SO 2 , C 6 H 14 , CH 4 , CO, C 3 H 8 , etc.

Micromachined nanocrystalline silver doped SnO2 H2S sensor

Semiconductor gas sensors using metal oxides, carbon nanotubes, graphene-based materials, and metal chalcogenides have been reviewed from the viewpoint of the sensitive, selective, and reliable detection of exhaled biomarker gases, and perspectives/strategies to realize breath analysis on a chip for disease diagnosis are discussed based on the concurrent design of high-performance sensing materials and miniaturized pretreatment components. Carbon-based sensing materials that show relatively high responses to NO and NH3 at low or mildly raised temperatures can be applied to the diagnosis of asthma and renal disease. Halitosis can be diagnosed by employing sensing or additive materials such as CuO and Mo that have high chemical affinities for H2S, while catalyst-loaded metal oxide nanostructure sensors or their arrays have been used to diagnose diabetes via the selective detection of acetone or by pattern recognition of sensor signals. For the ultimate miniaturization of a breath-analysis system into a tiny chip, preconditioning that includes preconcentration, dehumidification, and flow sensing needs to be either improved through the design of gas/moisture adsorbents or removed/simplified through the design of highly sensitive sensing materials that are less impervious to interference from humidity and temperature. Moreover, an abundant sensing library needs to be provided for the diagnosis of diseases (e.g. lung cancer) that are associated with multiple biomarker gases and for finding new methods to diagnose other diseases. For this aim, p-type oxide semiconductors with high catalytic activities, as well as combinatorial approaches, can be considered for the development of sensing materials that detect less-reactive large molecules, and high-throughput screening, respectively. Selectivity for a specific biomarker gas will simplify the system further. Breath analysis on a tiny chip using semiconductor chemiresistors with ultralow power consumption that is connected to the ‘Internet of Things’ will pave new roads for disease diagnosis and patient monitoring.

Toward breath analysis on a chip for disease diagnosis using semiconductor-based chemiresistors: recent progress and future perspectives

Deposition of ultrafine particle of silver on titanium dioxide (TiO2) significantly improved the photocatalytic activity for degradation of gaseous sulfur compounds, such as hydrogen sulfide (H2S) and methylmercaptan (CH3SH). The silver-deposited photocatalytic filter (Ag-PCF) was prepared by coating TiO2 powder on the porous ceramics substrate and successively depositing nano-sized silver particles on TiO2 by means of photodeposition method. The photocatalytic degradation rates of hydrogen sulfide and methylmercaptan by Ag-PCF were 7 times and 14 times larger, respectively, than those by conventional photocatalytic filter. The sulfur atom of hydrogen sulfide was oxidized into sulfate ion, which was accumulated on the filter. However, the degradation rate did not decline in the experimental period. The deposited silver seems to act as a co-catalyst that enhances the photocatalytic oxidation of sulfur compounds.

Photocatalytic degradation of gaseous sulfur compounds by silver-deposited titanium dioxide

H2S gas interaction mechanisms of sputtered SnO2 and SnO2–CuO bilayer sensors with a varying distribution of the Cu catalyst on SnO2 are studied using Pt interdigital electrodes within the sensing film. Sensitivity to H2S gas is investigated in the range 20–1200 ppm. Changes induced on the surface, the SnO2–CuO interface, and the internal bulk region of the sensing SnO2 film upon exposure to H2S have been analyzed to explain the increasing sensitivity of three different sensors SnO2, SnO2–CuO, and SnO2 with CuO islands. SnO2 film covered with 0.6 mm diameter ultrathin (∼10 nm) CuO dots is found to exhibit a high sensitivity of 7.3×103 at a low operating temperature of 150 °C. A response speed of 14 s for 20 ppm of H2S, and a fast recovery time of 118 s in flowing air have been measured. The presence of ultrathin CuO dotted islands allow effective removal of adsorbed oxygen from the uncovered SnO2 surface due to spillover of hydrogen dissociated from the H2S–CuO interaction, and the spillover mechanism is sensed through the observed fast response characteristics, and the high sensitivity of the SnO2–CuO-dot sensor.

Yuan-Jin He

Papers

Properties and mechanism study of Ag doped SnO2 thin films as H2S sensors