The noble metal nanoparticle has been widely utilized as a plasmonic unit to enhance biosensors, by leveraging its electric and/or optical properties. Integrated with the "flexible" feature, it further enables opportunities in developing healthcare products in a conformal and adaptive fashion, such as wrist pulse tracers, body temperature trackers, blood glucose monitors, etc. In this work, we present a holistic review of the recent advance of flexible plasmonic biosensors for the healthcare sector. The technical spectrum broadly covers the design and selection of a flexible substrate, the process to integrate flexible and plasmonic units, the exploration of different types of flexible plasmonic biosensors to monitor human temperature, blood glucose, ions, gas, and motion indicators, as well as their applications for surface-enhanced Raman scattering (SERS) and colorimetric detections. Their fundamental working principles and structural innovations are scoped and summarized. The challenges and prospects are articulated regarding the critical importance for continued progress of flexible plasmonic biosensors to improve living quality.

Flexible Plasmonic Biosensors for Healthcare Monitoring: Progress and Prospects.

Optical characterizations of PMMA/metal oxide nanoparticles thin films: bandgap engineering using a novel derived model.

This research was funded by the Portuguese Foundation for Science and Technology
(FCT) in the framework of the Strategic Funding UIDB/04650/2020; and by the project NANO4BIO
POCI-01-0145-FEDER-032299, with FCT reference PTDC/FISMAC/32299/2017. Marco S. Rodrigues
acknowledges FCT for his PhD Scholarship, SFRH/BD/118684/2016.

Gas Sensors Based on Localized Surface Plasmon Resonances: Synthesis of Oxide Films with Embedded Metal Nanoparticles, Theory and Simulation, and Sensitivity Enhancement Strategies

Gas sensing based on bulk refractive index (RI) changes, has been a challenging task for localized surface plasmon resonance (LSPR) spectroscopy, presenting only a limited number of reports in this field. In this work, it is demonstrated that a plasmonic thin film composed of Au nanoparticles embedded in a CuO matrix can be used to detect small changes (as low as 6 × 10-5 RIU) in bulk RI of gases at room temperature, using a high-resolution LSPR spectroscopy system. To optimize the film's surface, a simple Ar plasma treatment revealed to be enough to remove the top layers of the film and to partially expose the embedded nanoparticles, and thus enhance the film's gas sensing capabilities. The treated sample exhibits high sensitivity to inert gases (Ar, N2), presenting a refractive index sensitivity (RIS) to bulk RI changes of 425 nm/RIU. Furthermore, a 2-fold signal increase is observed for O2, showing that the film is clearly more sensitive to this gas due to its oxidizing nature. The results showed that the Au:CuO thin film system is a RI sensitive platform able to detect inert gases, which can be more sensitive to detect noninert gases as O2 or even other reactive species.

Optimization of Au:CuO Nanocomposite Thin Films for Gas Sensing with High-Resolution Localized Surface Plasmon Resonance Spectroscopy.

The present review attempts to comprehensively overview the progress in the field of nanoparticle-related analytical laser and plasma spectroscopy research, focusing on the results of the past decade. The discussion involves the brief description of the motivation and principle of operation behind all existing technologies. As a novel approach, the connection between nanoparticles and laser and plasma spectroscopy is discussed in all three major areas: monitoring of nanoparticle synthesis, nanoparticle characterization, as well as plasmonic signal enhancement achieved by using nanoparticles. In each area, a detailed description of methodological developments and modern applications is provided.

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Nanoparticles in analytical laser and plasma spectroscopy – a review of recent developments in methodology and applications

Magnetron sputtered nanocomposite thin films composed of monometallic Au and Ag, and bimetallic Au-Ag nanoparticles, dispersed in a CuO matrix, were prepared, characterized, and tested, which aimed to find suitable nano-plasmonic platforms capable of detecting the presence of gas molecules. The Localized Surface Plasmon Resonance phenomenon, LSPR, induced by the morphological changes of the nanoparticles (size, shape, and distribution), and promoted by the thermal annealing of the films, was used to tailor the sensitivity to the gas molecules. Results showed that the monometallic films, Au:CuO and Ag:CuO, present LSPR bands at ~719 and ~393 nm, respectively, while the bimetallic Au-Ag:CuO film has two LSPR bands, which suggests the presence of two noble metal phases. Through transmittance-LSPR measurements, the bimetallic films revealed to have the highest sensitivity to the refractive index changes, as well as high signal-to-noise ratios, respond consistently to the presence of a test gas.

/pdf/gas-sensing-with-nanoplasmonic-thin-films-composed-of-2o1nup0r0s.pdf

Gas Sensing with Nanoplasmonic Thin Films Composed of Nanoparticles (Au, Ag) Dispersed in a CuO Matrix

Optical gas sensors have emerged as an alternative to conventional conductometric sensors, opening new possibilities, especially for the detection of polluting and/or toxic molecules, such as carbon monoxide (CO). In this letter, an optimized plasmonic thin film, composed of Au nanoparticles embedded in a CuO matrix, was tested as a CO sensor, using high-resolution localized surface plasmon resonance (LSPR) spectroscopy. The Au:CuO thin film system was found to be highly sensitive, exhibiting a fast (∼14 s) and reversible (∼50 s) response when exposed to 50 ppm of CO, working at room temperature. Furthermore, the Au:CuO thin film prototype sensor presented a signal-to-noise ratio of 11, calculated at the transmittance minimum of the LSPR band, which allowed estimation of a detection limit as low as 14 ppm for CO gas.

Carbon Monoxide (CO) Sensor Based on Au Nanoparticles Embedded in a CuO Matrix by HR-LSPR Spectroscopy at Room Temperature

/pdf/chitosan-micro-membranes-with-integrated-gold-nanoparticles-lbppys2a.pdf

Chitosan Micro-Membranes with Integrated Gold Nanoparticles as an LSPR-Based Sensing Platform

In this study, thin films composed of gold nanoparticles embedded in a copper oxide matrix (Au:CuO), manifesting Localized Surface Plasmon Resonance (LSPR) behavior, were produced by reactive DC magnetron sputtering and post-deposition in-air annealing. The effect of low-power Ar plasma etching on the surface properties of the plasmonic thin films was studied, envisaging its optimization as gas sensors. Thus, this work pretends to attain the maximum sensing response of the thin film system and to demonstrate its potential as a gas sensor. The results show that as Ar plasma treatment time increases, the host CuO matrix is etched while Au nanoparticles are uncovered, which leads to an enhancement of the sensitivity until a certain limit. Above such a time limit for plasma treatment, the CuO bonds are broken, and oxygen is removed from the film’s surface, resulting in a decrease in the gas sensing capabilities. Hence, the importance of the host matrix for the design of the LSPR sensor is also demonstrated. CuO not only provides stability and protection to the Au NPs but also promotes interactions between the thin film’s surface and the tested gases, thereby improving the nanocomposite film’s sensitivity. The optimized sensor sensitivity was estimated at 849 nm/RIU, which demonstrates that the Au-CuO thin films have the potential to be used as an LSPR platform for gas sensors.

/pdf/optimization-of-au-cuo-thin-films-by-plasma-surface-21c9dyd8.pdf

Manuela Proença

Papers

Optimization of Au:CuO Nanocomposite Thin Films for Gas Sensing with High-Resolution Localized Surface Plasmon Resonance Spectroscopy.

Gas Sensing with Nanoplasmonic Thin Films Composed of Nanoparticles (Au, Ag) Dispersed in a CuO Matrix

Carbon Monoxide (CO) Sensor Based on Au Nanoparticles Embedded in a CuO Matrix by HR-LSPR Spectroscopy at Room Temperature

Chitosan Micro-Membranes with Integrated Gold Nanoparticles as an LSPR-Based Sensing Platform

Optimization of Au:CuO Thin Films by Plasma Surface Modification for High-Resolution LSPR Gas Sensing at Room Temperature