The analysis of volatile organic compounds in exhaled breath samples represents a new frontier in medical diagnostics because it is a noninvasive and potentially inexpensive way to detect illnesses. Clinical trials with spectrometry and spectroscopy techniques, the standard volatile-compound detection methods, have shown the potential for diagnosing illnesses including cancer, multiple sclerosis, Parkinson's disease, tuberculosis, diabetes, and more via breath tests. Unfortunately, this approach requires expensive equipment and high levels of expertise to operate the necessary instruments, and the tests must be done quickly and use preconcentration techniques, all of which impede its adoption. Sensing matrices based on nanomaterials are likely to become a clinical and laboratory diagnostic tool because they are significantly smaller, easier-to-use, and less expensive than spectrometry or spectroscopy. An ideal nanomaterial-based sensor for breath testing should be sensitive at very low concentrations of volatile organic compounds, even in the presence of environmental or physiological confounding factors. It should also respond rapidly and proportionately to small changes in concentration and provide a consistent output that is specific to a given volatile organic compound. When not in contact with the volatile organic compounds, the sensor should quickly return to its baseline state or be simple and inexpensive enough to be disposable. Several reviews have focused on the methodological, biochemical, and clinical aspects of breath analysis in attempts to bring breath testing closer to practice for comprehensive disease detection. This Account pays particular attention to the technological gaps and confounding factors that impede nanomaterial-sensor-based breath testing, in the hope of directing future research and development efforts towards the best possible approaches to overcome these obstacles. We discuss breath testing as a complex process involving numerous steps, each of which has several possible technological alternatives with advantages and drawbacks that might affect the performance of the nanomaterial-based sensors in a breath-testing system. With this in mind, we discuss how to choose nanomaterial-based sensors, considering the profile of the targeted breath markers and the possible limitations of the approach, and how to design the surrounding breath-testing setup. We also discuss how to tailor the dynamic range and selectivity of the applied sensors to detect the disease-related volatile organic compounds of interest. Finally, we describe approaches to overcome other obstacles by improving the sensing elements and the supporting techniques such as preconcentration and dehumidification.

Chemiresistors based on metal monolayer-capped nanoparticles (MCNPs) are promising candidates for fast, inexpensive, and portable tracing of (bio)chemical species in the gas phase. However, the sensitivity of such sensors to humidity is problematic, limiting their reliable and reproducible application in real-world environmental conditions. In this work, we employed a compensation method to explore the effect of humidity on a single MCNP chemiresistor as well as on an array of MCNP sensors used to analyze either synthetic or real-world samples. We show that an array of MCNP chemiresistors is able to precisely detect and estimate subtle concentrations of (mixtures of) volatile organic compounds (VOCs) under variable backgrounds of 2-83% relative humidity (RH) only after humidity compensation. Humidity effects were also tested in two clinical trials aimed at detecting prostate cancer and breast cancer through exhaled breath analysis. Analysis of the results showed improved cancer detection capabilities as a result of RH compensation, though less substantial than the impact of RH compensation on synthetic samples. This outcome is attributed to one - or a combination - of the following effects: (i) the RH variance was smaller in the breath samples than that in the synthetic samples; (ii) the VOC composition in the breath samples is less controlled than the synthetic samples; and (iii) the responses to small polar VOCs and water are not necessarily additive in breath samples. Ultimately, the results presented here could assist the development of a cost-effective, low-power method for widespread detection of VOCs in real-world applications, such as breath analysis, as well as for environmental, security, and food applications.

We report on the feasibility of cubic Pt nanoparticles (NPs) capped with four representative organic ligands, viz. oleylamine (ODA), 11-mercaptoundecanol, 11-mercaptoundecanoic acid, and benzylmercaptan, for sensing gaseous nonpolar analytes in humid atmospheres. Chemiresistors based on cubic Pt NPs with nonpolar ligands show a very large increase in resistance upon exposure to nonpolar analyte vapors, combined with a low sensitivity to polar analyte vapors, especially to water. The sensing mechanism can be understood in terms of analyte-induced changes in the NP-NP core distance and changes in the permittivity of the medium between the NPs. The sensing capabilities of the Pt NP chemiresistors for nonpolar molecules in highly humid atmospheres are demonstrated by dosing an ODA-capped cubic Pt NP sensor with air mixtures containing low octane concentrations and high humidity levels that are typical for many applications. The simple construction, low cost, stability, fast response, and high sensitivity to nonpolar molecules, together with the low sensitivity to water vapor, are promising features for sensing applications in real confounding atmospheres.

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Striving for cheap and robust manufacturing processes has prompted efforts to adapt and extend methods for printed electronics and biotechnology. A new “direct‐write” printing method for patterning nanometeric species in addressable locations has been developed, by means of evaporative deposition from a propelled anti‐pinning ink droplet (PAPID) in a manner analogous to a snail‐trail. Three velocity‐controlled deposition regimes have been identified; each spontaneously produces distinct and well‐defined self‐assembled deposition patterns. Unlike other technologies that rely on overlapping droplets, PAPIDs produce continuous patterns that can be formed on rigid or flexible substrates, even within 3D concave closed shapes, and have the ability to control the thickness gradient along the pattern. This versatile low cost printing method can produce a wide range of unusual electronic systems not attainable by other methods.

Nanoparticles play an integral part for the production of contacts and active sensing layers in the fast-developing printed electronic technology on flexible devices. Unfortunately, all currently available techniques for nanoparticle characterization are limited to ex situ and/or off-line processing. Here, we describe a new approach composed of two complementary parts for in situ and real-time estimation of the nanoparticles’ effective diameter on flexible substrates. The first part of the approach is based on measurements of electrical resistance of the device in response to strain, and correlation of the response with the nanoparticles’ diameter. The second part takes place only when measuring the electrical resistance is unfeasible. It is based on UV–vis absorption of the device and correlation of the absorption peak with the nanoparticle diameter based on previous calibration data from strain sensitivity. The new approach shows excellent estimations of the nanoparticle diameter (2.5–20 nm) on the substrate with the advantages of being online, in situ, and inexpensive. In addition, the estimated nanoparticle diameter is in excellent agreement with atomic force microscopy (AFM) measurements. These capabilities are expected to improve the process of “quality control” of the nanoscale-enabled flexible devices, which, until now, has been considered to be one of the most annoying issues that inhibits the commercialization of nanotechnology-based flexible products.