SERS active Ag/silicon based nanostructures for biosensing applications

L'abstract è presente nell'allegato / the abstract is in the attachmen

Role of probe design and bioassay configuration in surface enhanced Raman scattering based biosensors for miRNA detection

The accurate design of labelled oligo probes for the detection of miRNA biomarkers by Surface Enhanced Raman Scattering (SERS) may improve the exploitation of the plasmonic enhancement. This work, thus, critically investigates the role of probe labelling configuration on the performance of SERS-based bioassays for miRNA quantitation. To this aim, highly efficient SERS substrates based on Ag-decorated porous silicon/PDMS membranes are functionalized according to bioassays relying on a one-step or two-step hybridization of the target miRNA with DNA probes. Then, the detection configuration is varied to evaluate the impact of different Raman reporters and their labelling position along the oligo sequence on bioassay sensitivity. At high miRNA concentration (100-10 nM), a significantly increased SERS intensity is detected when the reporters are located closer to the plasmonic surface compared to farther probe labelling positions. Counterintuitively, a levelling-off of the SERS intensity from the different configurations is recorded at low miRNA concentration. Such effect is attributed to the increased relative contribution of Raman hot-spots to the whole SERS signal, in line with the electric near field distribution simulated for a simplified model of the Ag nanostructures. However, the beneficial effect of reducing the reporter-to-surface distance is partially retained for a two-step hybridization assay thanks to the less sterically hindered environment in which the second hybridization occurs. The study thus demonstrates an improvement of the detection limit of the two-step assay by tuning the probe labelling position, but sheds at the same time light on the multiple factors affecting the sensitivity of SERS-based bioassays

Microfluidic growth of Ag nanoparticles onto porous sllicon/PDMS surface for reliable SERS detection

Thanks to the miniaturized environment, microfluidics can offer an improved control over the growth of nanoparticles (NPs) particularly in terms of increased uniformity. An in-flow synthesis process can be therefore advantageous to fabricate plasmonic NPs for Surface-Enhanced Raman Scattering (SERS), which requires reproducible and uniform NPs arrangements aimed to obtain a repeatable Raman enhancement. In this study, silver NPs are synthesized in elastomeric microfluidic chips (polydimethylsiloxane, PDMS) hosting ultra-thin porous silicon (pSi) membranes. The NPs growth is investigated through the evolution of UV-Vis transmission spectra presenting Localized Surface Plasmon Resonances (LSPRs). The intrinsic reducing properties of the pSi towards the silver cations are exploited for the NPs synthesis. A motorized syringe is connected to the microfluidic chip and drives the injection of a AgNO3 solution into the reaction chamber (Fig.1a). Synthesis parameters, such as the silver nitrate concentration, the solvent, and the flow rate, are systematically varied in order to understand their influence on the NPs growth and morphology. At medium/high salt precursor concentrations (10-2 – 10-1 M) the appearance of a plasmonic dip over the pSi-PDMS spectral features is already evident after the first minute of reaction and a continuous red-shift combined with a broadening of the LSPRs is observed at increasing contact times until a saturation regime is reached at around 20 minutes (Fig1.b I-V). In order to compare the microfluidic results with static synthesis conditions, the immersion plating of an open pSi-PDMS membrane attached to a plastic cuvette is performed. In this case, a reduced growth rate is observed, as shown by the slower evolution of the plasmonic resonance (Fig1b VI), together with an altered morphology of the NPs. In fact, FESEM imaging reveals that under dynamic conditions the obtained NPs are smaller and more uniformly distributed on the pSi surface, in comparison to the larger and isolated particles obtained by static dipping, in agreement with their optical responses (Fig.b V-VII). Such differences are reflected in the SERS properties of the samples, which are tested with 4-mercaptobenzoic acid (4-MBA) as probe molecule. A high Raman signal intensity fluctuation is detected for the immersion plated substrates, while a higher average intensity and a better homogeneity characterize the in-flow synthesized ones. These results show the potentialities of the microfluidic process for the in situ fabrication of reliable SERS substrates

Microfluidic SERS chips for the selective detection of miRNAs in biological matrixes.

The employment of plasmonic nanostructures for Surface Enhanced Raman Scattering (SERS) based biosensing has been extensively explored, especially in the framework of early cancer diagnosis, which can benefit from the label-free nature and high sensitivity of SERS in a multiplexing approach. In this work, we discuss the detection of miRNA222, an important cancer biomarker, on silver decorated porous silicon-PDMS membranes integrated in elastomeric multichamber microfluidic chips, investigating the effectiveness of different analysis configurations for the determination of miRNAs in cell extracts. A bioassay based on a two-step hybridization procedure was developed (Fig.1a). According to this functionalization protocol, a DNA probe, complementary to the target sequence, is divided in two shorter strands (half1 and half2). Half1 is immobilized on the Ag nanoparticles (NPs), in order to specifically capture the target miRNA, then, half2, modified with a Raman label at the 5’ end, is incubated for the sensitive and label-free detection by SERS. After an accurate optimization of the protocol, its specificity for the target miRNA222 was confirmed by the analysis of mixtures of several miRNAs, even at high concentration, that simulate the effect of interfering sequences in real samples. In addition, a calibration curve was obtained, showing a decrease of the limit of detection for miRNA222, which was lowered of more than two orders of magnitude in comparison to the standard one-step hybridization protocol. Thanks to the new procedure, the analysis of cell extracts in the SERS microfluidic chip enabled the specific identification of the miRNA222 in the complex biological matrix. In order to further improve the sensitivity, different analysis configurations were finally investigated. The effect of different Raman dyes (Cyanine 5/3 or Rhodamine 6G) and of their labelling position (3’ or 5’) were studied. A significant boost of the SERS signal for the labelling at the 3’ end (close to the NPs surface) compared to the 5’ (far from the surface) was verified, due to a more efficient electromagnetic enhancement. The combination of this improved sensitivity with the high specificity of the assay paves the way to the effective application of these SERS chips in the early detection of tumor markers

SERS-active metal-dielectric nanostructures integrated in microfluidic devices for label-free quantitative detection of miRNA

In this work, SERS-based microfluidic PDMS chips integrating silver-coated porous silicon membranes were used for the detection and quantitation of microRNAs (miRNAs), which consist of short regulatory non-coding RNA sequences typically over-or under-expressed in connection with several diseases such as oncogenesis. In detail, metal-dielectric nanostructures which provide noticeable Raman enhancements were functionalized according to a biological protocol, adapted and optimized from an enzyme-linked immunosorbent assay (ELISA), for the detection of miR-222. Two sets of experiments based on different approaches were designed and performed, yielding a critical comparison. In the first one, the labelled target miRNA is revealed through hybridization to a complementary thiolated DNA probe, immobilized on the silver nanoparticles. In the second one, the probe is halved into shorter strands (half1 and half2) that interact with the complementary miRNA in two steps of hybridization. Such an approach, taking advantage of the Raman labelling of half2, provides a label-free analysis of the target. After suitable optimisation of the procedures, two calibration curves allowing quantitative measurements were obtained and compared on the basis of the SERS maps acquired on the samples loaded with several miRNA concentrations. The selectivity of the two-step assay was confirmed by the detection of target miR-222 mixed with different synthetic oligos, simulating the hybridization interference coming from similar sequences in real biological samples. Finally, that protocol was applied to the analysis of miR-222 in cellular extracts using an optofluidic multichamber biosensor, confirming the potentialities of SERS-based microfluidics for early-cancer diagnosis

Porous plasmonic nanostructures for Surface Enhanced Raman Scattering: development of biosensing platforms

82-85<span style="font-size:11.0pt;line-height:115%; font-family:" calibri","sans-serif";mso-ascii-theme-font:minor-latin;mso-fareast-font-family:="" "times="" new="" roman";mso-fareast-theme-font:minor-fareast;mso-hansi-theme-font:="" minor-latin;mso-bidi-font-family:"times="" roman";mso-ansi-language:en-us;="" mso-fareast-language:en-us;mso-bidi-language:ar-sa"="">The bundle tenacity and crystallite orientation of cotton fibres of 25 varieties of Gossypium barbadense species grown at Coimbatore farm of the Central Institute for Cotton Research in 1983 crop year were determined. The crystallite orientation was expressed in term of 40, 50 and 75% X-ray angles and the Hermans orientation factor, and the average orientation angle αm was computed from the latter. The bundle tenacity and the orientation parameters were correlated. A conclusion of the study is that the varieties should be characterized for strength on the basis of the Hermans factor and not the X-ray angles conventionally used.</span