Trends in Application of SERS Substrates beyond Ag and Au, and Their Role in Bioanalysis.

This article compares the applications of traditional gold and silver-based SERS substrates and less conventional (Pd/Pt, Cu, Al, Si-based) SERS substrates, focusing on sensing, biosensing, and clinical analysis. In recent decades plethora of new biosensing and clinical SERS applications have fueled the search for more cost-effective, scalable, and stable substrates since traditional gold and silver-based substrates are quite expensive, prone to corrosion, contamination and non-specific binding, particularly by S-containing compounds. Following that, we briefly described our experimental experience with Si and Al-based SERS substrates and systematically analyzed the literature on SERS on substrate materials such as Pd/Pt, Cu, Al, and Si. We tabulated and discussed figures of merit such as enhancement factor (EF) and limit of detection (LOD) from analytical applications of these substrates. The results of the comparison showed that Pd/Pt substrates are not practical due to their high cost; Cu-based substrates are less stable and produce lower signal enhancement. Si and Al-based substrates showed promising results, particularly in combination with gold and silver nanostructures since they could produce comparable EFs and LODs as conventional substrates. In addition, their stability and relatively low cost make them viable alternatives for gold and silver-based substrates. Finally, this review highlighted and compared the clinical performance of non-traditional SERS substrates and traditional gold and silver SERS substrates. We discovered that if we take the average sensitivity, specificity, and accuracy of clinical SERS assays reported in the literature, those parameters, particularly accuracy (93-94%), are similar for SERS bioassays on AgNP@Al, Si-based, Au-based, and Ag-based substrates. We hope that this review will encourage research into SERS biosensing on aluminum, silicon, and some other substrates. These Al and Si based substrates may respond efficiently to the major challenges to the SERS practical application. For instance, they may be not only less expensive, e.g., Al foil, but also in some cases more selective and sometimes more reproducible, when compared to gold-only or silver-only based SERS substrates. Overall, it may result in a greater diversity of applicable SERS substrates, allowing for better optimization and selection of the SERS substrate for a specific sensing/biosensing or clinical application.

Directly immersible silicon photonic probes: Application to rapid SARS-CoV-2 serological testing.

Silicon photonic probes based on broad-band Mach-Zehnder interferometry are explored for the first time as directly immersible immunosensors alleviating the need for microfluidics and pumps. Each probe includes two U-shaped waveguides allowing light in- and out-coupling from the same chip side through a bifurcated fiber and a mechanical coupler. At the opposite chip side, two Mach-Zehnder interferometers (MZI) are located enabling real-time monitoring of binding reactions by immersion of this chip side into a sample. The sensing arm windows of the two MZIs have different length resulting in two distinct peaks in the Fourier domain, the phase shift of which can be monitored independently through Fast Fourier Transform of the output spectrum. The photonic probes analytical potential was demonstrated through detection of antibodies against SARS-CoV-2 in human serum samples. For this, one MZI was functionalized with the Receptor Binding Domain (RBD) of SARS-CoV-2 Spike 1 protein, and the other with bovine serum albumin to serve as reference. The biofunctionalized probes were immersed for 10 min in human serum sample and then for 5 min in goat anti-human IgG Fc specific antibody solution. Using a humanized rat antibody against SARS-CoV-2 RBD, a detection limit of 20 ng/mL was determined. Analysis of human serum samples indicated that the proposed sensor discriminated completely non-infected/non-vaccinated from vaccinated individuals, and the antibodies levels determined correlated well with those determined in the same samples by ELISA. These results demonstrated the potential of the proposed sensor to serve as an efficient tool for expeditious point-of-care testing.

Ultrahigh Sensitive Au-Doped Silicon Nanomembrane Based Wearable Sensor Arrays for Continuous Skin Temperature Monitoring with High Precision.

Monitoring the body temperature with high accuracy provides a fast, facile, yet powerful route about the human body in a wide range of health information standards. Here, the first ever ultrasensitive and stretchable gold-doped silicon nanomembrane (Au-doped SiNM) epidermal temperature sensor array is introduced. The ultrasensitivity is achieved by shifting freeze-out region to intrinsic region in carrier density and modulation of fermi energy level of p-type SiNM through the development of a novel gold-doping strategy. The Au-doped SiNM is readily transferred onto an ultrathin polymer layer with a well-designed serpentine mesh structure, capable of being utilized as an epidermal temperature sensor array. Measurements in vivo and in vitro show temperature coefficient of resistance as high as -37270.72 ppm °C-1 , 22 times higher than existing metal-based temperature sensors with similar structures, and one of the highest thermal sensitivity among the inorganic material based temperature sensors. Applications in the continuous monitoring of body temperature and respiration rate during exercising are demonstrated with a successful capture of information. This work lays a foundation for monitoring body temperature, potentially useful for precision diagnosis (e.g., continuous monitoring body temperature in coronavirus disease 2019 cases) and management of disease relevance to body temperature in healthcare.

Surface Biochemical Modification of Poly(dimethylsiloxane) for Specific Immune Cytokine Response.

Recent evidence suggests that proinflammatory cytokines, such as tumor necrosis factor α (TNF-α), play a pivotal role in the development of inflammatory-related pathologies (covid-19, depressive disorders, sepsis, cancer, etc.,). More importantly, the development of TNF-α biosensors applied to biological fluids (e.g. sweat) could offer non-invasive solutions for the continuous monitoring of these disorders, in particular, polydimethylsiloxane (PDMS)-based biosensors. We have therefore investigated the biofunctionalization of PDMS surfaces using a silanization reaction with 3-aminopropyltriethoxysilane, for the development of a human TNF-α biosensor. The silanization conditions for 50 µm PDMS surfaces were extensively studied by using water contact angle measurements, electron dispersive X-ray and Fourier transform infrared spectroscopies, and fluorescamine detection. Evaluation of the wettability of the silanized surfaces and the Si/C ratio pointed out to the optimal silanization conditions supporting the formation of a stable and reproducible aminosilane layer, necessary for further bioconjugation. An ELISA-type immunoassay was then successfully performed for the detection and quantification of human TNF-α through fluorescent microscopy, reaching a limit of detection of 0.55 µg/mL (31.6 nM). Finally, this study reports for the first time a promising method for the development of PDMS-based biosensors for the detection of TNF-α, using a quick, stable, and simple biofunctionalization process.

Luminescent silicon nanostructures and COVID-19.

This Faraday Discussion volume is unique in the hundred plus year history of the Faraday Discussion series, being produced at a time of unprecedented circumstances worldwide and without the preceding Faraday Discussion conference having taken place.