Yaoli Zhao

Real-time detection of hazardous chemicals in water that appear at parts-per-trillion level concentrations with high sensitivity and selectivity is challenging. Achieving ultrahigh sensitivity using techniques based on immobilized receptors and electrochemistry requires the use of preconcentrators for sample enrichment, which imposes a time penalty. In addition, the use of immobilized receptors results in sensor-to-sensor irreproducibility challenges. Here, we demonstrate a real-time standoff technique for the ultrasensitive detection of chemical species in water based on a nanomechanical photothermal technique. This method involves exciting trace amounts of molecules in water by using tunable infrared light and directing the scattered light onto a cantilever, inducing mechanical deflections. The wavelength-dependent nanomechanical deflections were then used to quantify the contaminants in the water. We demonstrate this technique by selective detection of 1 part-per-trillion perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), and fluorotelomer alcohol (FTOH) in distilled and deionized water without using preconcentrators, labels, or receptors and demonstrate its functionality in industrial surface water samples. The proposed method also does not require sample preparation and is amenable to deployment in field applications.

The widespread industrial and consumer use of per- and polyfluoroalkyl substances (PFAS) has led to their persistent presence in the environment, driven by their robust carbon-fluorine bonds and bioaccumulative properties. This contamination poses serious health and ecological risks, making real-time, selective, and sensitive detection of PFAS critical for effective mitigation. We demonstrate a selective and sensitive detection of vapor-phase PFAS using photothermal cantilever deflection spectroscopy (PCDS), achieving a detection limit of ∼30 pg. This method eliminates the need for chemically selective coatings, relying instead on the physisorption of PFAS molecules onto a bi-material microcantilever. By leveraging mid-infrared absorption and monitoring both cantilever bending and resonance frequency, PCDS enables simultaneous chemical identification and mass quantification. The technique demonstrates high selectivity in the mid-infrared fingerprint region and rapid desorption of analytes, offering significant advantages for real-time environmental monitoring and public health protection.

Gallium‐based liquid metals (LMs) are widely used for stretchable and reconfigurable electronics thanks to their fluidic nature and excellent conductivity. These LMs possess attractive optical properties for photonics applications as well. However, due to the high surface tension of the LMs, it is challenging to form LM nanostructures with arbitrary shapes using conventional nanofabrication techniques. As a result, LM‐based nanophotonics has not been extensively explored. Here, a simple yet effective technique is demonstrated to deterministically fabricate LM nanopatterns with high yield over a large area. This technique demonstrates for the first time the capability to fabricate LM nanophotonic structures of various precisely defined shapes and sizes using two different LMs, that is, liquid gallium and liquid eutectic gallium–indium alloy. High‐density arrays of LM nanopatterns with critical feature sizes down to ≈100 nm and inter‐pattern spacings down to ≈100 nm are achieved, corresponding to the highest resolution of any LM fabrication technique developed to date. Additionally, the LM nanopatterns demonstrate excellent long‐term stability under ambient conditions. This work paves the way toward further development of a wide range of LM nanophotonics technologies and applications.

Molecular-level spectroscopy is crucial for sensing and imaging applications, yet detecting and quantifying minuscule quantities of chemicals remains a challenge, especially when they surface-adsorb in low numbers. Here, we introduce a photothermal spectroscopic technique that enables high selectivity sensing of adsorbates with an attogram detection limit. Our approach utilizes the Seebeck effect in a microfabricated nanoscale thermocouple junction, incorporated into the apex of a microcantilever. We observe minimal thermal mass exhibited by the sensor which maintains exceptional thermal insulation. The temperature variation driving the thermoelectric junction arises from the non-radiative decay of molecular adsorbates' vibrational states on the tip. We demonstrate the detection of photothermal spectra of physisorbed trinitrotoluene (TNT) and dimethyl methylphosphonate (DMMP) molecules, as well as representative polymers, with an estimated mass of 10^-18 g. Figure 1

The use of hydrogen as an energy carrier will require low‐cost, low‐power hydrogen sensors. Toward this goal, penta‐twinned palladium nanowires (Pd NWs) are synthesized and fabricated sensors from them by drop‐casting. Pd NWs drop‐cast onto an interdigitated electrode (IDE) gave a response of 0.3% to 1 vol.% H₂, with response and recovery times of 12 and 20 s, respectively. However, they exhibited a negative response (decreased resistance) at low H₂ concentrations. Pd NWs on a paper substrate provided a tenfold higher response to 1 vol.% H₂, with response and recovery times of 10 s each, but still exhibited negative response at low H₂ concentration. Exposing the Pd NW‐on‐paper sensor to ozone‐generating UV light degraded the PVP used in Pd NW synthesis, eliminating the reverse sensing response, and providing a response of 5% to 1 vol.% H₂, with response and recovery times of 15 s. This allowed reliable H₂ detection down to 100 ppm H₂. Finally, coating the Pd NWs with a small amount of Pt (2 dissociation. This work provides a path to low‐cost sensors to enable the safe use of H₂ as an energy carrier.

“This feature is intended to let ECS award-winning students and post-docs write primary author perspectives on their field, their work, and where they believe things are going. This month we highlight the work of Yaoli Zhao, the 2023 Sensor Division Graduate Student Award Winner.”

An accurate molecular identification of plastic waste is important in increasing the efficacy of automatic plastic sorting in recycling. However, identification of real-world plastic waste, according to their resin identification code, remains challenging due to the lack of techniques that can provide high molecular selectivity. In this study, a standoff photothermal spectroscopy technique, utilizing a microcantilever, was used for acquiring mid-infrared spectra of real-world plastic waste, including those with additives, surface contaminants, and mixed plastics. Analysis of the standoff spectral data, using Convolutional Neural Network (CNN), showed 100% accuracy in selectively identifying real-world plastic waste according to their respective resin identification codes. Standoff photothermal spectroscopy, together with CNN analysis, offers a promising approach for the selective characterization of waste plastics in Material Recovery Facilities (MRFs).

Molecular-level spectroscopy is crucial for sensing and imaging applications, yet detecting and quantifying minuscule quantities of chemicals remain a challenge, especially when they surface adsorb in low numbers. Here, we introduce a photothermal spectroscopic technique that enables the high selectivity sensing of adsorbates with an attogram detection limit. Our approach utilizes the Seebeck effect in a microfabricated nanoscale thermocouple junction, incorporated into the apex of a microcantilever. We observe minimal thermal mass exhibited by the sensor, which maintains exceptional thermal insulation. The temperature variation driving the thermoelectric junction arises from the nonradiative decay of molecular adsorbates' vibrational states on the tip. We demonstrate the detection of photothermal spectra of physisorbed trinitrotoluene (TNT) and dimethyl methylphosphonate (DMMP) molecules, as well as representative polymers, with an estimated mass of 10^-18 g.

Electrochromic pseudocapacitive transition-metal oxide materials, such as tungsten oxide, which combine fast response, high energy density, and optical effects, can play a significant role as energy storage materials. Here we investigate the electrochemical kinetics of thin films of tungsten oxide, which turn transparent to sky-blue color in the lithiated state due to the reduction of W ⁶⁺ to W ⁵⁺ . We investigated the charge density, charge transfer, ion diffusion, and interfacial behavior upon Li ⁺ insertion/de-insertion in WO ₃ . The electrochromic thin film’s pseudocapacitive and electrical double layer mechanism was differentiated based on the power-law. Faradaic diffusion-controlled process dominates over the surface capacitive behavior at scan rates below 40 mV s ⁻¹ . These films exhibit an areal charge density of around 100 mC cm ⁻² and a capacitance of 80 mF cm ⁻² , superior to most comparable electrochromic materials and supercapacitors. This work combines electrochromics and energy storage properties and provides a fundamental understanding of pseudocapacitive and electrochromic mechanisms in WO ₃ .

Standoff detection based on optical spectroscopy is an attractive method for identifying materials at a distance with very high molecular selectivity. Standoff spectroscopy can be exploited in demanding practical applications such as sorting plastics for recycling. Here, we demonstrate selective and sensitive standoff detection of polymer films using bi-material cantilever-based photothermal spectroscopy. We demonstrate that the selectivity of the technique is sufficient to discriminate various polymers. We also demonstrate in situ, point detection of thin layers of polymers deposited on bi-material cantilevers using photothermal spectroscopy. Comparison of the standoff spectra with those obtained by point detection, FTIR, and FTIR-ATR show relative broadening of peaks. Exposure of polymers to UV radiation (365 nm) reveal that the spectral peaks do not change with exposure time, but results in peak broadening with an overall increase in the background cantilever response. The sensitivity of the technique can be further improved by optimizing the thermal sensitivity of the bi-material cantilever and by increasing the number of photons impinging on the cantilever.