(Invited) Plasma Assisted Enhancement in Optical and Electronic Properties of Metal Nanoparticles: Application in Electrochemical Biosensing

Abstract

The sensitivity enhancement of biosensors can be achieved by increasing the active surface area using nanoparticle-functionalized electrodes. However, these enhancements have certain limitations due to a lack of information about the properties of nanoparticles and their behavior under certain conditions arising due to polydispersity, orientation, and surface roughness.Nanoparticles (NPs) have broad spectra of their applications in electronics. In most sensing methods, the deposition of high-density NPs causes polydispersity. Hence, the distribution achieved on the electrode surface conceals the effects of their size-dependent properties. Moreover, the tethered patterning of recognition molecules on the electrode surface is always represented using simple sketches, which fail to consider the non-idealities in the target-surface arrangement. The data obtained in these cases are averaged over these phenomena is challenging to interpret.The interparticle interactions play an active role in enhancing the sensitivity of the biosensors; these interactions are controlled by the Van der Waals force of attraction and the Electrostatic double layer (EDL) on the surface. According to DLVO mode, the Van der Waals force and EDL on the surface of nanoparticles are dependent on average nanoparticle size, the distance between interacting surfaces, absolute temperature, stern layer thickness, and zeta potential. In this work, we present an experimental setup to control the nanoparticle distribution and the surface charges. The experimental methods consist of depositing nanoparticles by drop-casting, aerosol spraying, and atmospheric plasma-assisted aerosolized deposition (Figure 1).The studies are carried out to investigate the impact of atmospheric plasma treatment on the morphology and optical properties of gold nanoparticles (Au NPs). The morphological changes such as size reduction observed due to the surface plasma treatment of AuNPs are attributed to the symmetric partial oxidation. The plasma deposited films were used as SERS substrate to detect R6G peaks. The intensity of the SERS signal of R6G from plasma-treated AuNP substrate was ~95 times as high as that of the untreated substrate. This enhancement is attributed to the surface roughness, presence of nanogaps, and size reduction after the treatment of NPs on the substrates. The morphological changes are explained by a redox reaction induced by the energy and concentration of reactive species in the plasma environment.Surface architectures of electrochemical biosensors have complex structures which facilitate the effective capture of probes (analytes). A minor defect in the architecture can cause an error in probe-target interaction. In such electrochemical sensors, the primary assumption made is that the sensor has an ideal architecture. Nevertheless, these sensors drive the detection of analytes as the focus is on the probe-target interaction. The underlying surface properties due to the resultant architecture are overlooked while analyzing the detection phenomenon. Hence, assessing the dependence of the sensing process on the non-idealities present is required to achieve reliable measurements.This study also presents the plasma-induced enhancement of electrical properties of Au NPs at room temperature. The distribution of plasma-assisted aerosolized deposited NPs showed the promising architecture for surface modification of the electrochemical sensor that has improved the electrochemical sensing, which is validated by detecting the cortisol molecules (cortisol is a stress biomarker found in interstitial fluids, saliva, and sweat). This improved electrochemical response is due to plasma-assisted surface activation of NPs. Another reason for the enhancement is the increase in the density of NPs on the substrate leading to a larger electroactive surface area (Figure 2A, 2B).These NP functionalized electrodes were then modified using DTSP SAM (dithiobis(succinimidyl propionate) self-assembled monolayer) and cortisol antibodies to detect cortisol molecules. The enhancement in the sensitivity was observed for plasma-assisted AuNPs functionalized electrodes as against drop-casted and aerosol sprayed electrodes (Figure 3A, 3B, 3C, 3D).In summary, a novel technique of plasma-assisted NPs deposition, which could maintain the inter-particle repulsive barrier to obtain the uniform monolayer of NPs, is reported. This barrier potential was achieved due to the de-agglomeration of NPs during deposition using nebulizer in experimental assembly and providing the surface excitation and surface charge tuning of NPs during plasma-assisted deposition. The properties of plasma-treated NP films at room temperature are comparable to the thermally-treated films at higher temperatures. This unique feature can be used for processing metal nanoparticles at room temperatures for designing sensitive SERS substrates on flexible materials such as polymers, papers, and textiles. Figure 1