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Owing to their extraordinary properties, carbon-based nanomaterials are gaining traction in biomedicine. Green synthesis is the cost-effective method for fabricating carbon-based nanomaterials due to its rapidity, renewable nature, and sustainability. This study emphasis on the graphene oxide (GO) reduction using a simple one-pot technique that does not require the use of toxic reducing agents. This article reports the green synthesis of reduced graphene oxide (RGO) using Hibiscus sabdariffa L. calyxes extract as the natural reducing agent. Additionally, this article also provides analysis RGO using X-ray diffraction (XRD), UV-Visible spectroscopy (UV-Vis), and Raman spectroscopy. XRD result showed that the GO peak at 11o diminished, and a new hump appear at 22o indicating that the GO is fully reduced when it is refluxed for 6 hours, at 100oC with 1:3 ratio of GO:PE. The UV-Vis data indicated absorption peak of GO (237 nm) and RGO (265 nm) at distinct locations. This finding shed new light on the enormous potential of Hibiscus sabdariffa L. calyxes extract for green GO reduction. As a result, this environmentally friendly method can help reduce dependence on chemical materials.

Stand-off Raman spectroscopy combines the advantages of both Raman spectroscopy and remote detection to retrieve molecular vibrational fingerprints of chemicals at inaccessible sites. However, it is currently restricted to the detection of pure solids and liquids and not widely applicable for dispersed molecules in air. Herein, we realize real-time stand-off SERS spectroscopy for remote and multiplex detection of atmospheric airborne species by integrating a long-range optic system with a 3D analyte-sorbing metal–organic framework (MOF)-integrated SERS platform. Formed via the self-assembly of Ag@MOF core–shell nanoparticles, our 3D plasmonic architecture exhibits micrometer thick SERS hotspot to allow active sorption and rapid detection of aerosols, gas, and volatile organic compounds down to parts-per-billion levels, notably at a distance up to 10 m apart. The platform is highly sensitive to changes in atmospheric content, as demonstrated in the temporal monitoring of gaseous CO2 in several cycles. Importantly, we demonstrate the remote and multiplex quantification of polycyclic aromatic hydrocarbon mixtures in real time under outdoor daylight. By overcoming core challenges in current remote Raman spectroscopy, our strategy creates an opportunity in the long-distance and sensitive monitoring of air/gaseous environment at the molecular level, which is especially important in environmental conservation, disaster prevention, and homeland defense. Ministry of Education (MOE) Nanyang Technological University Accepted version X.Y.L. thanks the Singapore Ministry of Education Tier 1 (RG11/18) and Tier 2 (MOE2016-T2-1-043) grants. G.C.P.-Q. and C.S.L.K. are thankful for Nanyang President’s Graduate Scholarships. N.Y. is thankful for the CN Yang scholarship. T.L. acknowledges the funding support from the National Natural Science Foundation of China (51433001).

While surface-enhanced Raman scattering (SERS) is a useful technique for the rapid and sensitive detection of biochemical compounds, conventional SERS chips suffer from high cost, complicated fabrication, inefficient sample collection processes and being not biocompatible. Here, we developed a facile, low-cost and highly sensitive gold nanostar (AuNS) immobilized paper-based SERS substrate that can be easily prepared in any laboratory. We performed studies on the paper materials, immobilization strategies, and SERS acquisition conditions to optimize the SERS enhancement and demonstrated that an optimized SERS signal was obtained from a dry substrate and wet analyte configuration suitable for rapid point-of-care detection. Using crystal violet (CV) as the Raman probe molecule, the optimized SERS substrate was prepared by having multiple drops of ∼100 pM of sodium citrate-treated colloidal AuNS on common laboratory filter paper before acquiring SERS spectra of CV freshly dripped onto the pre-dried AuNS-filter paper substrate. The optimized AuNS-filter paper substrate exhibited a SERS enhancement factor higher than that of two commercial Au/Ag-based SERS chips, with a detection limit of 1 nM CV and a SERS enhancement factor of up to 1.2 × 107. Such an optimized dry substrate and wet analyte configuration meant that the paper-based SERS substrate could be stored before use and Raman acquisition could be performed immediately without the need for the sample to dry. This makes the AuNS-filter paper substrate a simple and low-cost tool for trace level detection of biochemical species in a rapid, sensitive and non-destructive manner.

Molecular‐level airborne sensing is critical for early prevention of disasters, diseases, and terrorism. Currently, most 2D surface‐enhanced Raman spectroscopy (SERS) substrates used for air sensing have only one functional surface and exhibit poor SERS‐active depth. “Aerosolized plasmonic colloidosomes” (APCs) are introduced as airborne plasmonic hotspots for direct in‐air SERS measurements. APCs function as a macroscale 3D and omnidirectional plasmonic cloud that receives laser irradiation and emits signals in all directions. Importantly, it brings about an effective plasmonic hotspot in a length scale of approximately 2.3 cm, which affords 100‐fold higher tolerance to laser misalignment along the z‐axis compared with 2D SERS substrates. APCs exhibit an extraordinary omnidirectional property and demonstrate consistent SERS performance that is independent of the laser and analyte introductory pathway. Furthermore, the first in‐air SERS detection is demonstrated in stand‐off conditions at a distance of 200 cm, highlighting the applicability of 3D omnidirectional plasmonic clouds for remote airborne sensing in threatening or inaccessible areas. MOE (Min. of Education, S’pore) Accepted version

201 p. Long period fiber grating or LPG sensor has been gaining popularity due to its sensitivity to numerous perturbations such as temperature, strain and refractive index. The LPG has an advantage over other optical fiber sensors for refractive index sensing since the cladding region of the optical fiber, with LPG, is not required to be etched away in order to gain access to the evanescent field. Since the cladding modes propagates in the cladding within the LPG region of the optical fiber. The advantage is that the structural integrity of the optical fiber remains intact as compared to other optical fiber refractive index sensors with cladding etched away. Currently, a LPG sensor converts a measurand into wavelength. To interrogate the information would require expensive and complicated wavelength interrogation system. This will increase the overall system cost for industrial applications. In addition, other perturbations such as temperature and strain will affect the LPG wavelength, compromising the accuracy of the results. The aim of this research was the investigation of LPG for sensing applications such that the output from the LPG sensors is intensity-based instead of wavelength-based. DOCTOR OF PHILOSOPHY (EEE)

The one-pot seedless protocol provides a facile approach in the synthesis of gold nanostars (AuNS) that involves only three reagents, gold (III) chloride (HAuCl4), silver nitrate (AgNO3) and ascorbic acid (C6H8O6). While studies correlating the synthesis parameters of the seed-mediated protocol to surface-enhance Raman scattering (SERS) enhancement is well reported, the same understanding of the one-pot seedless protocol is limited. Here, we aim to elucidate how the synthesis parameters of AuNS from the one-pot seedless protocol, the AuNS concentration, surface passivation and aggregation level affect the colloidal SERS enhancement. Using crystal violet (CV) as a Raman probe molecule, we found that the SERS enhancement increases with Au3+/C6H8O6 molar ratio up to 0.60 and Au3+/Ag+ molar ratio up to 18. Although the surfactant, cetyltrimethylammonium bromide (CTAB) maintained colloidal stability, it reduced the SERS enhancement. Interestingly, the SERS enhancement did not increase monotonically with AuNS concentration, but decreased when AuNS concentration was beyond 15 pM. The SERS enhancement also increased with the increasing level of salt-induced aggregation of AuNS, but only within a few minutes. While the concept of SERS with colloidal nanostructures is not new, we have shown for the first time, a detailed systematic study of various parameters that affect the SERS enhancement of AuNS synthesized using a one-pot seedless protocol. This study enables us to optimize the SERS enhancement of AuNS at the synthesis level to make them effective colloid-based SERS substrates for potential use in intracellular biosensing.