Plasmonic Nanosensors: Design, Fabrication, and Applications in Biomedicine

Current advances in the fabrication of smart nanomaterials and nanostructured surfaces find wide usage in the biomedical field. In this context, nanosensors based on localized surface plasmon resonance exhibit unprecedented optical features that can be exploited to reduce the costs, analytic times, and need for expensive lab equipment. Moreover, they are promising for the design of nanoplatforms with multiple functionalities (e.g., multiplexed detection) with large integration within microelectronics and microfluidics. In this review, we summarize the most recent design strategies, fabrication approaches, and bio-applications of plasmonic nanoparticles (NPs) arranged in colloids, nanoarrays, and nanocomposites. After a brief introduction on the physical principles behind plasmonic nanostructures both as inherent optical detection and as nanoantennas for external signal amplification, we classify the proposed examples in colloid-based devices when plasmonic NPs operate in solution, nanoarrays when they are assembled or fabricated on rigid substrates, and nanocomposites when they are assembled within flexible/polymeric substrates. We highlight the main biomedical applications of the proposed devices and offer a general overview of the main strengths and limitations of the currently available plasmonic nanodevices.

Application of a thermo-sensitive imprinted SERS substrate to the rapid trace detection of ofloxacin

A novel composite (AgNPs-MIPs) was prepared by combining nano-silver particles with an ofloxacin (OFL) imprinted thermo-sensitive hydrogel. The thermo-sensitive optical properties of the composite were studied and it was used as a Raman substrate for the detection of ofloxacin. The results have shown that the position and intensity of the plasmon resonance absorption peak of the AgNPs-MIPs can be reversibly changed with the change of temperature, and the intensity of the ofloxacin Raman signal increases with the increase of temperature. Because the hydrogel combined Raman enhancement of silver nanoparticles, the selectivity of molecularly imprinted materials and the intelligent response of thermo-sensitive hydrogels, it can realize rapid, in situ, trace and selective detection of ofloxacin. Moreover, the detection limit can reach 10^-10 mol L^-1.

Single-Step Photochemical Formation of Near-Infrared-Absorbing Gold Nanomosaic within PNIPAm Microgels: Candidates for Photothermal Drug Delivery

This work demonstrates the dynamic potential for tailoring the surface plasmon resonance (SPR), size, and shapes of gold nanoparticles (AuNPs) starting from an Au(I) precursor, chloro(dimethyl sulfide)gold (I) (Au(Me2S)Cl), in lieu of the conventional Au(III) precursor hydrogen tetrachloroaurate (III) hydrate (HAuCl4). Our approach presents a one-step method that permits regulation of an Au(I) precursor to form either visible-absorbing gold nanospheres or near-infrared-window (NIRW)-absorbing anisotropic AuNPs. A collection of shapes is obtained for the NIR-absorbing AuNPs herein, giving rise to spontaneously formed nanomosaic (NIR-absorbing anisotropic gold nanomosaic, NIRAuNM) without a dominant geometry for the tesserae elements that comprise the mosaic. Nonetheless, NIRAuNM exhibited high stability; one test sample remains stable with the same SPR absorption profile 7 years post-synthesis thus far. These NIRAuNM are generated within thermoresponsive poly(N-isopropylacrylamide) (PNIPAm) microgels, without the addition of any growth-assisting surfactants or reducing agents. Our directed-selection methodology is based on the photochemical reduction of a light-, heat-, and water-sensitive Au(I) precursor via a disproportionation mechanism. The NIRAuNM stabilized within the thermoresponsive microgels demonstrates a light-activated size decrease of the microgels. On irradiation with a NIR lamp source, the percent decrease in the size of the microgels loaded with NIRAuNM is at least five times greater compared to the control microgels. The concept of photothermal shrinkage of hybrid microgels is further demonstrated by the release of a model luminescent dye, as a drug release model. The absorbance and emission of the model dye released from the hybrid microgels are over an order of magnitude higher compared to the absorbance and emission of the dye released from the unloaded-control microgels.

Tunable Electromagnetic Coupling in Plasmonic Nanostructures Mediated by Thermoresponsive Polymer Brushes

A smart and highly SERS-active plasmonic platform was designed by coupling regular arrays of nanotriangles to colloidal gold nanorods via a thermoresponsive polymer spacer (poly(N-isopropylacrylamide), PNIPAM). The substrates were prepared by combining a top-down and a bottom-up approach based on nanosphere lithography, surface-initiated controlled radical polymerization, and colloidal assembly. This multistep strategy provided regular hexagonal arrays of nanotriangles functionalized by polymer brushes and colloidal gold nanorods, confined exclusively on the nanotriangle surface. Interestingly, one could finely tune the gold nanorod impregnation on the polymer-coated nanostructures by adjusting the polymer layer thickness, leading to highly coupled plasmonic systems for intense SERS signal. Moreover, the thermoresponsive properties of the PNIPAM brushes could be wisely handled in order to monitor the SERS activity of the nanostructures coupled via this polymer spacer. The coupled hybrid plasmonic nanostructures designed in this work are therefore very promising smart platforms for the sensitive detection of analytes by SERS.

A Surface-Enhanced Raman Scattering Sensor Integrated with Battery-Controlled Fluidic Device for Capture and Detection of Trace Small Molecules

For surface-enhanced Raman scattering (SERS) sensors, one of the important issues is the development of substrates not only with high SERS-activity but also with strong ability to capture analytes. However, it is difficult to achieve the two goals simultaneously especially when detecting small molecules. Herein a compact battery-controlled nanostructure-assembled SERS system has been demonstrated for capture and detection of trace small molecule pollutants in water. In this SERS fluidic system, an electrical heating constantan wire covered with the vertically aligned ZnO nanotapers decorated with Ag-nanoparticles is inserted into a glass capillary. A mixture of thermo-responsive microgels, Au-nanorods colloids and analyte solution is then filled into the remnant space of the capillary. When the system is heated by switching on the battery, the thermo-responsive microgels shrink, which immobilizes the analyte and drives the Au-nanorod close to each other and close to the Ag-ZnO nanotapers. This process has also created high-density “hot spots” due to multi-type plasmonic couplings in three-dimensional space, amplifying the SERS signal. This integrated device has been successfully used to measure methyl parathion in lake water, showing a great potential in detection of aquatic pollutants.

Hybrid nanostructures for SERS: materials development and chemical detection

This review focuses on recent developments in hybrid and nanostructured substrates for SERS (surface-enhanced Raman scattering) studies. Thus substrates composed of at least two distinct types of materials, in which one is a SERS active metal, are considered here aiming at their use as platforms for chemical detection in a variety of contexts. Fundamental aspects related to the SERS effect and plasmonic behaviour of nanometals are briefly introduced. The materials described include polymer nanocomposites containing metal nanoparticles and coupled inorganic nanophases. Chemical approaches to tailor the morphological features of these substrates in order to get high SERS activity are reviewed. Finally, some perspectives for practical applications in the context of chemical detection of analytes using such hybrid platforms are presented.

Silver-gelatine bionanocomposites for qualitative detection of a pesticide by SERS

Gelatine based nanocomposites incorporating Ag nanoparticles as a new SERS platform for the detection of diethyldithiocarbamate (EtDTC), aiming controlled release applications.

Three-dimensional plasmonic hydrogel architecture: facile synthesis and its macroscale effective space

A facile strategy was developed to fabricate a 3D hydrogel decorated with Ag nanoparticles as a SERS substrate. The macro effective depth in this 3D network was confirmed. The substrate produced satisfactory results in the analysis of trace environmental molecules.

Supramolecular hybrid hydrogel based on host-guest interaction and its application in drug delivery

In this work, we developed a simple, novel method for constructing gold nanocomposite supramolecular hybrid hydrogels for drug delivery, in which gold nanocrystals were utilized as building blocks. First, methoxypoly(ethylene glycol) thiol (mPEG-SH, molecular weight (MW)=5 K) capped gold nanocrystals (nanospheres and nanorods) were prepared via a facile one-step ligand-exchange procedure. Then, the homogeneous supramolecular hybrid hydrogels were formed, after adding α-cyclodextrin (α-CD) into PEG-modified gold nanocrystal solutions, due to the host-guest inclusion. Both gold nanoparticles and inclusion complexes formed between α-CD and PEG chain provided the supra-cross-links, which are beneficial to the gelation formation. The resulting hybrid hydrogels were fully characterized by a combination of techniques including X-ray diffraction, rheology studies, and scanning electron microscopy. Meanwhile, the hybrid hydrogel systems demonstrated unique reversible gel-sol transition properties at a certain temperature caused by the temperature-responsive reversible supramolecular assembly. The drug delivery applications of such hybrid hydrogels were further investigated in which doxorubicin was selected as a model drug for in vitro release, cytotoxicity, and intracellular release studies. We believe that the development of such hybrid hydrogels will provide new and therapeutically useful means for medical applications.

Composite blends of gold nanorods and poly(t-butylacrylate) beads as new substrates for SERS

Polymer based composites containing metal nanoparticles are shown to provide new substrates for SERS detection and simultaneously enable the development of new tools for molecular sensing. A very important aspect on the use of hybrid materials relates to the observation of synergistic effects that result from the use of distinct components such as a polymer and metal nanoparticles. We report here the preparation of new SERS substrates made from blends of colloidal Au (nanospheres and nanorods) and PtBA (poly(t-butylacrylate)). The observed SERS enhancement depends on the characteristics of the obtained hybrid material. When compared to the starting Au colloids, the Au/PtBA nanocomposites led to SERS spectra with more intense bands, using 2,2'-dithiodipyridine as molecular probe. The origin of the stronger Raman signal in this case, is possibly due to a combination of events related to the nanocomposite characteristics, including the formation of Au assemblies at the polymer surface due to particle clustering, and the absorption increase in a spectral region closer to the laser excitation wavelength. The strategy described here is a low-cost process with potential for the up-scale fabrication of SERS substrates, namely by using other types of nanocomposites.

Multifunctional microgel magnetic/optical traps for SERS ultradetection

We report on the fabrication of a SERS substrate comprising magnetic and silver particles encapsulated within a poly(N-isopropylacrylamide) (pNIPAM) thermoresponsive microgel. This colloidal substrate has the ability to adsorb analytes from solution while it is expanded (low temperature) and reversibly generate hot spots upon collapse (high temperature or drying). Additionally, the magnetic functionality permits concentration of the composite particles into small spatial regions, which can be exploited to decrease the amount of material per analysis while improving its SERS detection limit. Proof of concept for the sequestration of uncommon molecular systems is demonstrated through the first SERS analysis of pentachlorophenol (PCP), a chlorinated ubiquitous environmental pollutant.