We fabricate SERS sensors by inkjet printing and demonstrate that their SERS response correlates with their diffuse reflectance characteristics. Using a modified commercial inkjet-printer, SERS sensors are prepared with multiple printing passes. Performances of the printed sensors only become noticeable after five printing passes as observed in both SERS and diffuse reflectance measurements. This suggests that the simpler diffuse reflectance measurement can be used as an alternative method to characterize and optimize the SERS performance of the printed sensors. Although sensors with a very high number of printing passes exhibit a much stronger SERS response from the benzenethiol reporter molecule, we also noticed a significant increase in the background from blank sensors. This may not be a desirable feature particularly for the detection of weakly bound molecules. Controlling SERS background and attaining a desirable SERS enhancement would need to be balanced in the design of sensors for the end-user’s specific need.
The development of rapid and sensitive detection technology for identifying of chemicals and biological agents such as contraband substances, narcotics and toxins is critical for decision-making among first responders and military personnel. Recent advances in nanofabrication, microelectronics and computational power have led to miniaturization of portable analytical instruments. Among these, handheld Raman analyzer coupled with Surface Enhanced Raman spectroscopy (SERS), have become increasingly common for field detection challenges due to the enormous sensitivity of SERS technique. In this paper, we demonstrate the fabrication and analysis of flexible and porous paper-based SERS sensors by inkjet printing of colloidal Au nanoparticles (AuNP) onto paper substrate. Our paper-based SERS sensors are cost-effective and robust, and they provide the added advantage of point-of-sampling capability that rigid SERS sensors lack. With their inherent filtration sampling capability, we coupled our paper-SERS sensors with air pump for active sampling and detection of chemical aerosols. Additionally, we printed the SERS sensors in test strip format to enable swab sampling of chemical contaminants on door handle as a simulated field-sampling and detection of chemical toxins. Our swab sampling successfully picked up enough benzenethiol, BPE and fentanyl molecules to trigger positive detection. The used swab can also be preserved for further confirmatory tests such as paper-spray mass spectrometry.
Inkjet-printed surface enhanced Raman spectroscopy (SERS) sensors are fabricated on cellulose based paper or fabric substrates. These flexible sensors provides basic point-of-sampling advantages that is particularly useful in field applications. Due to the heterogeneous loading of nanoparticles on the substrate, SERS intensities inevitably vary across the active area of the printed sensor. This paper will discuss the use of receiver operating characteristics (ROC) for the analysis of inkjet-printed SERS sensors. The aim is to provide an alternative measurand to the SERS enhancement factor that can be used to compare different types of SERS substrates. We have developed statistical analysis from multiple data sets obtained from sensors exposed to both analyte and control to determine the probability of positive detection (PD) at various analyte concentration. This dependence describes the ROC of the sensor and also provides confidence level associated with a given detection limit. We propose this methodology for the evaluation of SERS sensors to enable their field applications.
In this study, we will present the synthesis of self-assembled coupled Au nanorods (NRs) as substrates capable of supporting a dual modality of surface enhanced spectroscopies, SERS and SEIRAS. The AuNR arrays can be assembled either through vertical alignment or lateral alignment. We will present different assembly strategies for the Au NRs by adjusting the ionic strength of the Au NR solution. The goal is to rely on self-assembly to create organized and reproducible sensors for small molecule detection. Field enhancement criteria differs between SERS and SEIRAS. We will also present the finite-difference time-domain (FDTD) simulation of the multilayered AuNR array across visible and SWIR spectral region to explain some of the experimental observations.
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