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The utilization of UV-Vis spectroscopy with amino-functionalized carbon quantum dots (NCQD) as a positive fluorophore reagent for chloride sensing in oil marks a notable advancement in analytical spectroscopy chemistry. This approach streamlines the detection process by eliminating the need for lengthy procedures and pretreatment steps typically associated with chloride detection in edible oil. By incorporating NCQD in chloride detection within the oil matrix, the wavelength analysis transitions from the UV to the visible region. This shift eliminates interference from oil matrix interactions, ensuring more accurate results. Molecular analysis of NCQD reveals significant shifts in its Fourier Transformation Infrared and photoluminescence spectroscopy peaks due to interaction with chloride in edible oil. It has two impressive sensitivity ranges spanning from 0.1-1.0 to 1.0-8.0 ppm, with a value of -0.4656 au. ppm -1 (R 2  = 0.998) and -0.0361 au. ppm -1 (R 2  = 0.931), respectively, the technique meets regulatory standards while achieving a low limit of detection (LOD) of 0.1 ppm. This places it on par with conventional methods and commercial sensors. The NCQD-UV-Vis spectroscopy method not only enhances the efficiency and accuracy of chloride detection but also holds promise for various industrial applications requiring simple and precise monitoring of chloride levels in oil samples., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Surface plasmonic sensors have received considerable attention, found extensive applications, and outperformed conventional optical sensors. In this work, biopolymer chitosan (CS) was used to prepare the bilayer structure (CS/Au) of a plasmonic refractive index sensor for dopamine (DA) detection. The sensing characteristics of the developed plasmonic sensor were evaluated. Increasing DA concentrations significantly shifted the SPR dips. The sensor exhibited stability and a refractive index sensitivity of 8.850°/RIU in the linear range 0.1 nM to 1 µM with a detection limit of 0.007 nM and affinity constant of 1.383 × 10 8 M -1 . The refractive index and thickness of the CS/Au structure were measured simultaneously by fitting the obtained experimental findings to theoretical data based on Fresnel equations. The fitting yielded the refractive index values n (1.5350 ± 0.0001) and k (0.0150 ± 0.0001) for the CS layer contacting 0.1 nM of DA, and the thickness, d was (15.00 ± 0.01) nm. Then, both n and d values increased by increasing DA concentrations. In addition, the changes in the FTIR spectrum and the variations in sensor surface roughness and structure obtained by AFM analysis confirmed DA adsorption on the sensing layer. Based on these observations, CS/Au bilayer has enhanced the performance of this plasmonic sensor, which showed promising importance as a simple, low-cost, and reliable platform for DA sensing.

In this study, graphene quantum dots (GQDs) and polyvinyl alcohol (PVA) composite was prepared and then coated on the surface of gold thin film via the spin coating technique. Subsequently, Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM), and ultraviolet-visible spectroscopy (UV-Vis) were adopted to understand the structure, surface morphology, and optical properties of the prepared samples. The FT-IR spectral analysis revealed important bands, such as O-H stretching, C=O stretching, C-H stretching, and O=C=O stretching vibrations. The surface roughness of the GQDs-PVA composite thin film was found to be increased after exposure to carbaryl. On the other hand, the optical absorbance of the GQDs-PVA thin film was obtained and further analysis was conducted, revealing a band gap E g value of 4.090 eV. The sensing potential of the thin film was analyzed using surface plasmon resonance (SPR) spectroscopy. The findings demonstrated that the developed sensor's lowest detection limit for carbaryl was 0.001 ppb, which was lower than that previously reported, i.e., 0.007 ppb. Moreover, other sensing performance parameters, such as full width at half maximum, detection accuracy, and signal-to-noise ratio, were also investigated to evaluate the sensor's efficiency.

Abnormality of dopamine (DA), a vital neurotransmitter in the brain's neuronal pathways, causes several neurological diseases. Rapid and sensitive sensors for DA detection are required for early diagnosis of such disorders. Herein, a carbon quantum dot (CQD)-based refractive index surface plasmon resonance (SPR) sensor was designed. The sensor performance was evaluated for various concentrations of DA. Increasing DA levels yielded blue-shifted SPR dips. The experimental findings revealed an excellent sensitivity response of 0.138°/pM in a linear range from 0.001 to 100 pM and a high binding affinity of 6.234 TM -1 . The effects of varied concentrations of DA on the optical characteristics of CQD thin film were further proved theoretically. Increased DA levels decreased the thickness and real part of the refractive index of CQD film, according to fitting results. Furthermore, the observed reduction in surface roughness using AFM demonstrated that DA was bound to the sensor layer. This, in turn, explained the blue shift in SPR reflectance curves. This optical sensor offers great potential as a trustworthy solution for direct measurement due to its simple construction, high sensitivity, and other sensing features.

Due to the crucial role of dopamine (DA) in health and peripheral nervous systems, it is particularly important to develop an efficient and accurate sensor to monitor and determine DA concentrations for diagnostic purposes and diseases prevention. Up to now, using surface plasmon resonance (SPR) sensors in DA determination is very limited and its application still at the primary stage. In this work, a simple and ultra-sensitive SPR sensor was constructed for DA detection by preparation of chitosan- graphene quantum dots (CS-GQDs) thin film as the sensing layer. Other SPR measurements were conducted using different sensing layers; GQDs, CS for comparison. The proposed thin films were prepared by spin coating technique. The developed CS-GQDs thin film-based SPR sensor was successfully tested in DA concentration range from 0 fM to 1 pM. The designed SPR sensor showed outstanding performance in detecting DA sensitively (S = 0.011°/fM, R 2  = 0.8174) with low detection limit of 1.0 fM has been achieved for the first time. The increased angular shift of SPR dip, narrow full width half maximum of the SPR curves, excellent signal-to-noise ratio and figure of merit, and a binding affinity constant (K A ) of 2.962 PM -1 demonstrated the potential of this sensor to detect DA with high accuracy. Overall, it was concluded that the proposed sensor would serve as a valuable tool in clinical diagnostic for the serious neurological disorders. This in turns has a significant socio-economic impact., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

In this study, X-ray photoelectron spectroscopy (XPS) was used to study chitosan-graphene oxide (chitosan-GO) incorporated with 4-(2-pyridylazo)resorcinol (PAR) and cadmium sulfide quantum dot (CdS QD) composite thin films for the potential optical sensing of cobalt ions (Co 2+ ). From the XPS results, it was confirmed that carbon, oxygen, and nitrogen elements existed on the PAR-chitosan-GO thin film, while for CdS QD-chitosan-GO, the existence of carbon, oxygen, cadmium, nitrogen, and sulfur were confirmed. Further deconvolution of each element using the Gaussian-Lorentzian curve fitting program revealed the sub-peak component of each element and hence the corresponding functional group was identified. Next, investigation using surface plasmon resonance (SPR) optical sensor proved that both chitosan-GO-based thin films were able to detect Co 2+ as low as 0.01 ppm for both composite thin films, while the PAR had the higher binding affinity. The interaction of the Co 2+ with the thin films was characterized again using XPS to confirm the functional group involved during the reaction. The XPS results proved that primary amino in the PAR-chitosan-GO thin film contributed more important role for the reaction with Co 2+ , as in agreement with the SPR results.

The optical constants of Para-Toluene sulfonic acid-doped polyaniline (PANI), PANIchitosan composites, PANI-reduced graphene-oxide composites and a ternary composite comprising of PANI, chitosan and reduced graphene-oxide dispersed in diluted p-toluene sulfonic acid (PTSA) solution and N -Methyl-2-Pyrrolidone (NMP) solvent have been evaluated and compared. The optical constant values were extracted from the absorbance spectra of thin layers of the respective samples. The potential utilization of the materials as the active sensing materials of surface plasmon resonance biosensors has also been assessed in terms of the estimated value of the penetration depth through a dielectric medium. The results show a reasonable dependence of the optical constant parameters on the solvent type. Higher real part refractive index ( n ) and real part complex dielectric permittivity ( ε ') values were observed for the samples prepared using PTSA solution, while higher optical conductivity values were observed for the NMP-based samples due to their relatively higher imaginary part refractive index ( k ) and imaginary part complex dielectric permittivity ( ε ″) values. In addition, NMP-based samples show improvement in terms of the penetration depth through a dielectric medium by around 9.5, 1.6, 4.4 and 2.9 times compared to PTSA-based samples for the PANI, PANI-chitosan, PANI-RGO and the ternary composites, respectively. Based on these, it is concluded that preparation of these materials using different dispersion solvents could produce materials of different optical properties. Thus, the variation of the dispersion solvent will allow the flexible utilization of the PANI and the composites for diverse applications.

In this study, synthesis and characterization of chitosan/maghemite (Cs/Fe 2 O 3 ) composites thin film has been described. Its properties were characterized using Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and ultraviolet-visible spectroscopy (UV-Vis). FTIR confirmed the existence of Fe-O bond, C-N bond, C-C bond, C-O bond, O=C=O bond and O-H bond in Cs/Fe 2 O 3 thin film. The surface morphology of the thin film indicated the relatively smooth and homogenous thin film, and also confirmed the interaction of Fe 2 O 3 with the chitosan. Next, the UV-Vis result showed high absorbance value with an optical band gap of 4.013 eV. The incorporation of this Cs/Fe 2 O 3 thin film with an optical-based method, i.e., surface plasmon resonance spectroscopy showed positive response where mercury ion (Hg 2+ ) can be detected down to 0.01 ppm (49.9 nM). These results validate the potential of Cs/Fe 2 O 3 thin film for optical sensing applications in Hg 2+ detection.

The modification of graphene quantum dots (GQDs) may drastically enhance their properties, therefore resulting in various related applications. This paper reported the preparation of novel cetyltrimethylammonium bromide/hydroxylated graphene quantum dots (CTAB/HGQDs) thin film using the spin coating technique. The properties of the thin film were then investigated and studied. The functional groups existing in CTAB/HGQDs thin film were confirmed by the Fourier transform infrared (FTIR) spectroscopy, while the atomic force microscope (AFM) displayed a homogenous surface of the thin film with an increase in surface roughness upon modification. Optical characterizations using UV-Vis absorption spectroscopy revealed a high absorption with an optical band gap of 4.162 eV. Additionally, the photoluminescence (PL) spectra illustrated the maximum emission peak of CTAB/HGQDs thin film at a wavelength of 444 nm. The sensing properties of the as-prepared CTAB/HGQDs thin film were studied using a surface plasmon resonance technique towards the detection of several heavy metal ions (HMIs) (Zn 2+ , Ni 2+ , and Fe 3+ ). This technique generated significant results and showed that CTAB/HGQDs thin film has great potential for HMIs detection.

In this study, the incorporation between gold modified-tyrosinase (Tyr) enzyme based graphene oxide (GO) thin film with surface plasmon resonance (SPR) technique has been developed for the detection of phenol. SPR signal for the thin film contacted with phenol solution was monitored using SPR technique. From the SPR curve, sensitivity, full width at half maximum (FWHM), detection accuracy (DA) and signal-to-noise ratio (SNR) have been analyzed. The sensor produces a linear response for phenol up to 100 µM with sensitivity of 0.00193° µM -1 . Next, it can be observed that deionized water has the lowest FWHM, with a value of 1.87° and also the highest value of DA. Besides, the SNR of the SPR signal was proportional to the phenol concentrations. Furthermore, the surface morphology of the modified thin film after exposed with phenol solution observed using atomic force microscopy showed a lot of sharp peaks compared to the image before in contact with phenol proved the interaction between the thin film and phenol.

In this work, sensitive detection of dengue virus type 2 E-proteins (DENV-2 E-proteins) was performed in the range of 0.08 pM to 0.5 pM. The successful DENV detection at very low concentration is a matter of concern for targeting the early detection after the onset of dengue symptoms. Here, we developed a SPR sensor based on self-assembled monolayer/reduced graphene oxide-polyamidoamine dendrimer (SAM/NH 2 rGO/PAMAM) thin film to detect DENV-2 E-proteins. Surface characterizations involving X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) confirms the incorporation of NH 2 rGO-PAMAM nanoparticles in the prepared sensor films. The specificity, sensitivity, binding affinity, and selectivity of the SPR sensor were then evaluated. Results indicated that the variation of the sensing layer due to different spin speed, time incubation, and concentration provided a better interaction between the analyte and sensing layer. The linear dependence of the SPR sensor showed good linearity (R 2  = 0.92) with the lowest detection of 0.08 pM DENV-2 E-proteins. By using the Langmuir model, the equilibrium association constant was obtained at very high value of 6.6844 TM -1 (R 2  = 0.99). High selectivity of the SPR sensor towards DENV-2 E-proteins was achieved in the presence of other competitors.

Surface plasmon resonance (SPR) sensors as novel optical sensors for the detection of a variety of analytes have been receiving increasing attention and their sensitivity has become the research hotspot recently. In this study, the sensitivity of an SPR optical sensor was enhanced by modifying a gold thin film with a nanocrystalline cellulose (NCC)-based material for zinc ion (Zn 2+ ) detection that exists in the environment due to industrial processing. By replacing the gold thin film with a novel modified-gold thin film, Zn 2+ can be detected from the range of 0 to 10 ppm using SPR. It is believed that the Zn 2+ may interact with the negative charge molecules that exist on the modified-gold thin film, and this was confirmed via X-ray photoelectron spectroscopy (XPS). Moreover, this modified-gold-SPR has a high sensitivity of 1.892° ppm -1 up to 0.1 ppm with an enhanced detection of Zn 2+ as low as 0.01 ppm. The SPR results also followed the Langmuir isotherm model with a binding affinity of 1.927 × 10 3 M -1 , which further confirmed the sensitivity of the SPR sensor. In addition, using the modified-gold thin film, SPR has a higher affinity towards Zn 2+ compared to other metal ions, i.e. Ni 2+ , Fe 2+ , Cr 2+ , Mn 2+ , and Co 2+ ., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

In this study, the incorporation of surface plasmon resonance (SPR) spectroscopy with novel chitosan-graphene oxide/cadmium sulphide quantum dots (CdS QDs) active layer for cobalt ion (Co 2+ ) detection has been developed. The interaction of different Co 2+ concentrations with the novel modified active layer was monitored using the SPR technique. From the SPR results, detection range, sensitivity, full width at half maximum (FWHM), detection accuracy (DA) and signal-to-noise ratio (SNR) have been analysed. The results showed the detection range of this optical sensor was 0.01 to 10 ppm, and it was saturated for higher concentration of Co 2+ . The sensitivity obtained was 0.1188 ppm -1 for low concentration of Co 2+ ranged from 0.01 to 1 ppm. The FWHM and DA were consistent for all concentration of Co 2+ , while the SNR of the SPR signal increased with the Co 2+ concentration. The SPR angle shifts were also fitted using Langmuir, Freundlich and Sips (Langmuir-Freundlich) isotherm models, where Sips model fitted the best with the binding affinity of 0.939 ppm -1 . The results proved that the novel chitosan-graphene oxide/CdS QDs modified gold thin film can detect Co 2+ via SPR spectroscopy.

About 71% of the Earth's surface is covered with water. Human beings, animals, and plants need water in order to survive. Therefore, it is one of the most important substances that exist on Earth. However, most of the water resources nowadays are insufficiently clean, since they are contaminated with toxic metal ions due to the improper disposal of pollutants into water through industrial and agricultural activities. These toxic metal ions need to be detected as fast as possible so that the situation will not become more critical and cause more harm in the future. Since then, numerous sensing methods have been proposed, including chemical and optical sensors that aim to detect these toxic metal ions. All of the researchers compete with each other to build sensors with the lowest limit of detection and high sensitivity and selectivity. Graphene quantum dots (GQDs) have emerged as a highly potential sensing material to incorporate with the developed sensors due to the advantages of GQDs. Several recent studies showed that GQDs, functionalized GQDs, and their composites were able to enhance the optical detection of metal ions. The aim of this paper is to review the existing, latest, and updated studies on optical sensing applications of GQDs-based materials toward toxic metal ions and future developments of an excellent GQDs-based SPR sensor as an alternative toxic metal ion sensor.

The emergence of unintentional poisoning and uncontrolled vector diseases have contributed to sensor technologies development, leading to the more effective detection of diseases. In this study, we present the combination of graphene-based material with surface plasmon resonance technique. Two different graphene-based material sensor chips were prepared for rapid and quantitative detection of dengue virus (DENV) and cobalt ion (Co 2+ ) as an example of typical metal ions. As the fundamental concept of surface plasmon resonance (SPR) sensor that relies on the refractive index of the sensor chip surface, this research focused on the SPR signal when the DENV and Co 2+ interact with the graphene-based material sensor chip. The results demonstrated that the proposed sensor-based graphene layer was able to detect DENV and Co 2+ as low as 0.1 pM and 0.1 ppm respectively. Further details in the detection and quantification of analyte were also discussed in terms of sensitivity, affinity, and selectivity of the sensor.

Surface plasmon resonance (SPR) is a label-free optical spectroscopy that is widely used for biomolecular interaction analysis. In this work, SPR was used to characterize the binding properties of highly sensitive nanocrystalline cellulose-graphene oxide based nanocomposite (CTA-NCC/GO) towards nickel ion. The formation of CTA-NCC/GO nanocomposite has been confirmed by FT-IR. The SPR analysis result shows that the CTA-NCC/GO has high binding affinity towards Ni 2+ from 0.01 until 0.1 ppm with binding affinity constant of 1.620 × 10 3  M -1 . The sensitivity for the CTA-NCC/GO calculated was 1.509° ppm -1 . The full width at half maximum (FWHM), data accuracy (DA), and signal-to-noise ratio (SNR) have also been determined using the obtained SPR curve. For the FWHM, the value was 2.25° at 0.01 until 0.08 ppm and decreases to 2.12° at 0.1 until 10 ppm. The DA for the SPR curves is the highest at 0.01 until 0.08 ppm and lowest at 0.1 until 10 ppm. The SNR curves mirrors the curves of SPR angle shift where the SNR increases with the Ni 2+ concentrations. For the selectivity test, the CTA-NCC/GO has the abilities to differentiate Ni 2+ in the mixture of metal ions., (Copyright © 2018 Elsevier B.V. All rights reserved.)

In this research, surface plasmon resonance (SPR) spectroscopy was used for sensing copper ion by combining the SPR with nanocrystalline cellulose modified by hexadecyltrimethylammonium bromide and graphene oxide composite (CTA-NCC/GO) thin film. The binding of Cu 2+ on CTA-NCC/GO thin film was monitored by using SPR spectroscopy. By using the obtained SPR curve, detection range, binding affinity, sensitivity, full width at half maximum (FWHM), data accuracy (DA), and signal-to-noise ratio (SNR) have been calculated. The results showed that the sensor detection range was 0.01 until 0.5 ppm, and that it reached a saturation value. Moreover, the resonance angle shift followed the Langmuir isotherm model with a binding affinity constant of 4.075 × 10 3 M -1 . A high sensitivity of 3.271° ppm -1 also was obtained for low Cu 2+ concentration ranged from 0.01 to 0.1 ppm. For the FWHM, the lowest value calculated was at 0.08 and 0.1 ppm, which is 3.35°. The DA of the SPR signal consecutively highest at 0.08 and 0.1 ppm. Besides that, the SNR of the SPR signal increases with the Cu 2+ concentrations. The CTA-NCC/GO thin film morphological properties were also studied by using atomic force microscopy. The rms roughness values, which were obtained before and after in contact with Cu 2+, were 3.51 nm and 2.46 nm, respectively.