基礎講座 電気泳動(Electrophoresis)

The main challenge for all electrical, mechanical and optical sensors is to detect low molecular weight (less than 400 Da) chemical and biological analytes under extremely dilute conditions. Surface plasmon resonance sensors are the most commonly used optical sensors due to their unique ability for real-time monitoring the molecular binding events. However, their sensitivities are insufficient to detect trace amounts of small molecular weight molecules such as cancer biomarkers, hormones, antibiotics, insecticides, and explosive materials which are respectively important for early-stage disease diagnosis, food quality control, environmental monitoring, and homeland security protection. With the rapid development of nanotechnology in the past few years, nanomaterials-enhanced surface plasmon resonance sensors have been developed and used as effective tools to sense hard-to-detect molecules within the concentration range between pmol and amol. In this review article, we reviewed and discussed the latest trend and challenges in engineering and applications of nanomaterials-enhanced surface plasmon resonance sensors (e.g., metallic nanoparticles, magnetic nanoparticles, carbon-based nanomaterials, latex nanoparticles and liposome nanoparticles) for detecting “hard-to-identify” biological and chemical analytes. Such information will be viable in terms of providing a useful platform for designing future ultrasensitive plasmonic nanosensors.

Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications

This paper reviews fundamentals of optical affinity biosensors based on plasmonic nanostructures and discusses recent advances in the development of this technology, including plasmonic nanostructures and surface plasmon phenomena, advances in sensor instrumentation, and functional coatings. Examples of applications for both the detection of chemical and biological substances as well as the investigation of biomolecular interactions are also given.

Optical Biosensors Based on Plasmonic Nanostructures: A Review

Whispering gallery modes (WGMs) have been exploited for a broad range of sensing applications. However, the vast majority of WGM sensors consist of passive resonators, requiring complex interrogation systems to be employed, ultimately limiting their practicality. Active resonators containing a gain medium, allowing remote excitation and collection of the WGM-modulated fluorescence spectra, have emerged as an alternative to passive resonators. Although research is still in its infancy, recent progress has reduced the performance gap between the two paradigms, fueled by the potential for new applications that could not previously be realized. Here, recent developments in sensors based on active WGM microresonators are reviewed, beginning with a discussion of the theory of fluorescence-based and lasing WGMs, followed by a discussion of the variety of gain media, resonator architectures, and emerging sensing applications. We conclude with a discussion of the prospects and future directions for improving active WGM sensors.

Fluorescent and lasing whispering gallery mode microresonators for sensing applications

The red shift and the broadening of the Raman signal from microcrystalline silicon films is described in terms of a relaxation in the q-vector selection rule for the excitation of the Raman active optical phonons. The relationship between width and shift calculated from the known dispersion relation in c-Si is in good agreement with available data. An increase in the decay rate of the optical phonons predicted on the basis of the same model is confirmed experimentally.

The one phonon raman spectrum in microcrystalline silicon

Surface enhanced Raman spectroscopy (SERS) has established itself as a promising tool in optical sensing technology. Efforts have been made to improve practicalities of the technology with regards to costs of production, stability, reproducibility, flexibility and robustness. Here, we demonstrate a method to fabricate Ag/Au bimetallic inverted nanopyramid (i-NPyr) and upright nanopyramid (u-NPyr) using a multi-step molding process on flexible plastics as SERS sensors for the ultrasensitive detection of haemoglobin at a very low concentration (down to nM). First, a Si i-NPyr master was created using electron beam lithography, then i-NPyr and u-NPyr structures were imprinted on flexible polymer substrates using nanoimprinting lithography, and last, Ag/Au coatings were coated on top of them. The SERS activity of the imprinted i-NPyr and u-NPyr substrates were evaluated using rhodamine 6G (Rh6G) as probe molecules. Enhancement factor (EF) values of 3.88 x 10^6 and 7.86 x 10^5 respectively for the iNPyr and u-NPyr substrates, and the i-NPyr SERS substrate was able to detect Rh6G at concentrations as low as 1pM. In addition, we investigated the stability and resilience of i-NPyr by thoroughly analysing signal performance under angled bending and delicate crumpling. Finally, a proof-of-concept application as a wearable i-NPyr SERS sensor was demonstrated by detecting the analyte in sweat excreted during running and walking. We believe that this extremely sensitive i-NPyr SERS substrate with dense electric field confinement around hotspots (confirmed using computational simulations), as well as its good stability, durability, and low production cost, may enable in-situ measurement of analytes in wearable technologies.

Ag/Au coated inverted nanopyramids as flexible and wearable SERS substrates for biomolecular sensing

In this work, we investigated both spontaneous and stimulated whispering gallery mode (WGM) emissions of 2 mol. % Li+-doped ZnO (Li-ZnO) microspheres with different sizes under 325 and 488 nm wavelength laser excitations, respectively. It was found that all the microspheres exhibit stimulated emissions under a visible laser excitation source of 488 nm wavelength after the threshold pumping power. Thereafter, we studied the dependence of threshold pumping power on the size of microresonators to achieve stimulated emissions by individual microspheres. Furthermore, two microspheres (MS2 and MS3) are excited via a 325 nm UV laser, and surprisingly, the WGM peaks of higher intensity are observed in the visible rather than in the UV spectral region. We expected that most of the emissions are achieved via defect states transitions instead of inter-band transitions in the microresonators. It was found that WGMs in each microsphere exhibit a linear spectral shift of 3–5 nm with increasing pumping power of 488 nm excitation laser source. We believe that these proposed microspheres can be utilized effectively as WGM-based visible lasers and sensors.

Investigation of size-dependent spontaneous and stimulated visible WGM emissions via both ultraviolet and visible excitations for sensing applications

We experimentally investigated Zinc oxide (ZnO) nanowires (NWs) on flat Si substrate and ZnO NWs on Au
nanoislands attached on a Si substrate via hydrothermal technique, pursuing surface enhanced Raman scattering (SERS).
Au nanoislands were formed by thermal annealing of a Au thin film deposited on Si substrate. ZnO NWs were then
grown on two types of substrates (with and without Au nanoislands) and thermally annealed together. During the thermal
annealing process, the ZnO NWs were coupled to Au nanoislands. After the thermal annealing, strong SERS
enhancement was observed of ZnO NWs on Au nanoislands. Over 30 times enhancement in SERS was found when the
initial Au layer thickness was 40 nm.

Huge SERS enhancement via ZnO nanowires on gold nanoislands

Thermal nanoimprint lithography based plasmonic nanogratings for refractive index sensing of polar solvents

In this work, a label-free and inexpensive method for the monitoring of water pollutants is demonstrated. We introduce a localized surface plasmon resonance (LSPR) based plasmonic capillary optical biosensor to detect microalgae cells. Here, the plasmonic capillary biosensor was prepared by decorating the inner walls of a glass capillary with gold nanoparticles that were employed for investigations. Since the gold nanoparticle has the potential to sense pollutants in water rapidly with high sensitivity and they are expected to perform a significant role in environmental monitoring. Our proposed plasmonic capillary sensor has a detection limit of 25 algal cells (Chlorella sp. CB4). Furthermore, the plasmonic capillary sensing platform significantly simplifies sensor fabrication and reduces the cost of the device. We believe that the presented plasmonic sensor could stand as a potential candidate for developing a cost-effective, label-free, and rapid sensing platform to detect microalgae pollutants present in the water at very low concentrations.

Rakesh S. Moirangthem

Papers

Ag/Au coated inverted nanopyramids as flexible and wearable SERS substrates for biomolecular sensing

Investigation of size-dependent spontaneous and stimulated visible WGM emissions via both ultraviolet and visible excitations for sensing applications

Huge SERS enhancement via ZnO nanowires on gold nanoislands

Thermal nanoimprint lithography based plasmonic nanogratings for refractive index sensing of polar solvents

Portable Capillary Sensor Integrated with Plasmonic Platform for Monitoring Water Pollutants