We fabricate SERS sensors by inkjet printing of colloidal Au nanoparticles on porous paper substrates. Using a modified commercial inkjet-printer, SERS sensors are prepared with multiple printing passes. SERS response is correlated with their diffuse reflectance characteristics. Chemical analyte detection is only possible with both SERS and diffuse reflectance measurements from sensors that has been subjected to more than 5 printing passes. 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. Sensors with a higher number of printing passes exhibit a much stronger SERS response from strong adsorbing analytes such as benzenethiol molecule. We compare the performances of 8 and 15 printing passes sensors with the commercially available paper-based sensors, p-SERS. We calculate their Relative Enhancement Factor (REF) by comparing their performances to the first order phonon vibration of Si, which serves as a reference standard. Lastly, we demonstrate the use of such sensors for the detection of chemical aerosols.
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.
An accurate supervised classification technique requires a large training database with an equal number of samples in each category. However, in practice, data class imbalance is naturally inherent in detection and identification tasks. In an extreme case, one category of data has a majority of training samples (positive class), causing over-classifying. In these circumstances, the negative classes are either absent, poorly sampled or not well defined. Deep one-class classifiers are artificial neural networks developed to overfit the positive class samples. This unique situation constrains the network model to be trained data features just with the knowledge of the positive class. One well-known application of one-class classifiers is for anomaly detection problem, where the model stands out outliers. In this study, we proposed using a one-dimensional CNN model for anomaly detection of Surface-Enhanced Raman Spectra applicable for Portable Raman Spectrometer in field investigations.
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.
The plasmon resonance of noble metal nanoparticles (NP) manifests itself in a variety of extraordinary optical properties.
Resonant excitation of the conduction electrons by incident radiation generates a localized surface plasmon resonance
(LSPR) that is responsible for a variety of surface enhanced optical phenomena. This unique optical property coupled
with well-established surface chemistry allows us to utilize both Ag and Au NP as optical contrasting agents to probe
and monitor the surface receptors of cells. We have employed two plasmon-assisted optical techniques (namely, surface
enhanced Raman scattering, and resonant Rayleigh scattering) to monitor the adrenergic receptors in mammalian
cardiomyocyte cells that have been labeled with functionalized Ag NPs. In this study, a unique Raman reporter
molecule, 4-(mercaptomethyl)benzonitrile, was developed to provide an easily identifiable vibration, the C≡N stretch, in
a spectral window free from Raman bands of cell constituents and other biomolecules used in receptor crosslinking and
surface passivation. Successfully labeled cells were then monitored with both optical techniques. Both techniques are
related through the plasmonic properties of the noble metal NP and combined with high resolution imaging techniques;
we outline the importance that different NP architectures play in the different imaging techniques. Furthermore, we will
discuss the instrumentation and plasmonic implications in the design of NP best suited for such multimodal imaging
approaches.
Multi-modal sensing scheme significantly improves the detection accuracy but can also introduce
extra complexity in the overall design of the sensor. We overcome this difficulty by utilizing the
plasmonic properties of metallic nanoparticles. In this study, we will present a simple dual optical
sensing mechanism which harvests signals of the resonantly excited metallic nanostructure in the
form of surface enhanced Raman scattering (SERS) and resonant Rayleigh scattering. Silver and
gold nanoparticles labeled with appropriate antibodies act as signal transduction units and upon
exposure to the targeted pathogen render the targeted species optically active. We demonstrate that
detection of a single pathogen cell is easily attainable with the dual detection scheme. Furthermore,
we explore the markedly different SERS intensity observed from the use of two very different
antibody recognition units during the pathogen labeling process.
Raman spectroscopy has demonstrated to be an effective tool in the detection and classification of pathogenic microorganisms. The technique is, however, limited by the inherently low cross-section of the Raman scattering process. Among the many enhanced Raman processes, surface enhanced Raman scattering (SERS) technique provides the highest sensitivity and can be easily adapted in the bio-sensing applications such as DNA hybridization and protein binding events. In this study, we report the targeted detection of the pathogenic bacteria, Staphylococcus aureus, with novel single domain antibody (sdAb) conjugated SERS nanoprobes. A sdAb specific to protein A of S. aureus cells was conjugated to silver nanoparticles (Ag-NP). Bacteria recognition was achieved through specific binding of the sdAb (conjugated to SERS nanoprobe) to protein A. Binding rendered the nanoparticle-labeled S. aureus cells SERS active. As a result, S. aureus cells could be detected rapidly and with excellent sensitivity by monitoring the SERS vibrational signatures. This work demonstrates that the SERS imaging technique offers excellent sensitivity with a detection limit of a single bacterium.
Silver nanoparticles (Ag-NPs) functionalized with molecule probes which contains unique Raman vibrational mode as
well as a recognition binding site to target specific surface proteins expressed on the transfected human cervical cancel
cells was utilized to detect the cell surface protein through surface enhanced Raman scattering (SERS) microscopy. In
this study, we demonstrate that only the aggregated Ag-NPs displays detectable SERS signal. We also observed striking
polarization anisotropy in many dimer or trimer NP aggregates. This work will impact on the future design of NP
clusters for in-vivo cell imaging.
Photoluminescent porous Si (pSi) is a potentially attractive material for biosensor devices. Its ease of fabrication, large active surface area and unique optical properties are just some important attributes. Among other transduction techniques, it is possible to monitor the onset of molecular binding events through the effective quenching of the bright pSi photoluminescence. Here we present the study of effective quenching through a colloidal Ag nanoparticle interaction with pSi. Placing the metallic nanoparticles in close proximity to the light emitting pSi can effectively sweep away the charge carriers from the semiconductor surface and result in a carrier depletion region near the Si-nanoparticle interface. By labeling the targeted bio-species with a silver nanoparticle, and the pSi surface with an appropriate receptor molecule ; in-situ PL monitoring can provide a real-time transduction scheme for the pSi- based biosensor devices.
Recently, the development of Si-based optical sensors for protein and other biochemicals has become of great interest. Here, we examine the protein and Si-based substrate interaction by studying the BSA interaction with surface derivatized porous Si (pSi). The pSi fabricated through electrochemical anodization of crystalline silicon in hydrofluoric acid showed an average pore diameter of ~ 10 nm. Chemically functionalization of pSi by thermal reaction with undecylenic acid produced an organic monolayer covalently attached to the silicon surfaces. Bovine serum albumin (BSA) was then adsorbed onto the acid-terminated pSi surfaces. The resulting surfaces were characterized using scanning electron microscopy (SEM), ellipsometry and Fourier transform infrared spectroscopy (FTIR). Ellipsometry and SEM both showed that the BSA molecule penetrated more than 1 μm into the porous structure. SEM further revealed the damaged and partially lifted-off porous film from the silicon substrate after a prolonged BSA adsorption. It is caused by the BSA penetrating deep into the porous structure and anchoring itself tightly through strong electrostatic interaction with the acid-covered pSi sidewalls. A change in surface tension during BSA film formation then causes the pSi layer to buckle and lift-off from the underlying Si substrate. FTIR results from the undecylenic acid-modified pSi surfaces after BSA adsorption showed strong characteristic Amide I, II and III vibrational bands. The role of the surface chemistry, wetting properties, substrate porosity and topography will be discussed.
Recent results suggest that surface-enhanced Raman Spectroscopy (SERS) of single adsorbate molecules is possible under appropriate circumstances. We propose that this phenomenon is associated with very intense enhancements available at interstitial sites (hot spots) of nanoparticle assemblies (either colloid particle aggregates or rough surfaces) illuminated with light of an appropriate wavelength so as to excite surface plasmons, coupled with additional resonance enhancements due to a judicious choice of ad-molecule. The former contribution, known as electromagnetic (EM) enhancement, has been known for years to be capable of producing EM hot spots where the enhancement can top 1011. This fact seems to have been rediscovered recently. It is also known that the fields at the surface of fractal aggregates commonly show hot spots. These are also, at times, capable of such high local enhancements. On fractals, the location of these hot spots are, however, highly dependent on parameters such as the excitation wavelength. In contrast, small compact clusters (when properly designed) have the benefit of a wavelength-independent hot spot where a small number of molecules could be (chemically) directed and detected. This insight suggests an eventual optimally engineered single-molecule SERS system with predictable enhancement capabilities and optimal adsorption (i.e. chemical) characteristics at the hot spot.
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