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The development of carbon dots (CDs), either using green or chemical precursors, has inevitably led to their wide range application, from bioimaging to optoelectronic devices. The reported precursors and properties of these CDs have opened new opportunities for the future development of high-quality CDs and applications. Green precursors were classified into fruits, vegetables, flowers, leaves, seeds, stem, crop residues, fungi/bacteria species, and waste products, while the chemical precursors were classified into acid reagents and non-acid reagents. This paper quickly reviews ten years of the synthesis of CDs using green and chemical precursors. The application of CDs as sensing materials in optical sensor techniques for environmental monitoring, including the detection of heavy metal ions, phenol, pesticides, and nitroaromatic explosives, was also discussed in this review. This profound review will offer knowledge for the upcoming community of researchers interested in synthesizing high-quality CDs for various applications.

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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.

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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, R2 = 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 (KA) 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.

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Insecticides are enormously important to industry requirements and market demands in agriculture. Despite their usefulness, these insecticides can pose a dangerous risk to the safety of food, environment and all living things through various mechanisms of action. Concern about the environmental impact of repeated use of insecticides has prompted many researchers to develop rapid, economical, uncomplicated and user-friendly analytical method for the detection of insecticides. In this regards, optical sensors are considered as favorable methods for insecticides analysis because of their special features including rapid detection time, low cost, easy to use and high selectivity and sensitivity. In this review, current progresses of incorporation between recognition elements and optical sensors for insecticide detection are discussed and evaluated well, by categorizing it based on insecticide chemical classes, including the range of detection and limit of detection. Additionally, this review aims to provide powerful insights to researchers for the future development of optical sensors in the detection of insecticides.

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In this paper, the structural and optical properties of ZnO-SiO2-based ceramics fabricated from oil palm empty fruit bunch (OPEFB) were investigated. The OPEFB waste was burned at 600, 700 and 800 °C to form palm ash and was then treated with sulfuric acid to extract silica from the ash. X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses confirmed the existence of SiO2 in the sample. Field emission scanning electron microscopy (FESEM) showed that the particles displayed an irregular shape and became finer after leaching. Then, the solid-state method was used to produce the ZnO-SiO2 composite and the samples were sintered at 600, 800, 1000, 1200 and 1400 °C. The XRD peaks of the Zn2SiO4 showed high intensity, which indicated high crystallinity of the composite. FESEM images proved that the grain boundaries were larger as the temperature increased. Upon obtaining the absorbance spectrum from ultraviolet-visible (UV-Vis) spectroscopy, the energy band gaps obtained were 3.192, 3.202 and 3.214 eV at room temperature, 600 and 800 °C, respectively, and decreased to 3.127, 2.854 and 2.609 eV at 1000, 1200 and 1400 °C, respectively. OPEFB shows high potential as a silica source in producing promising optical materials.

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Arecanut husk (AH) was selected as a material for silica replacement in the synthesis process of glass-ceramics zinc silicate and also the fact that it has no traditional use and often being dumped and results in environmental issues. The process of pyrolysis was carried out at temperature 700 °C and above based on thermogravimetric analysis to produce arecanut husk ash (AHA). The average purity of the silica content in AHA ranged from 29.17% to 45.43%. Furthermore, zinc oxide was introduced to AHA and zinc silicate started to form at sintering temperature 700 °C and showed increased diffraction intensity upon higher sintering temperature of 600 °C to 1000 °C based on X-ray diffraction (XRD) analysis. The grain sizes of the zinc silicate increased from 1011 nm to 3518 nm based on the morphological studies carried out by field emission scanning electron microscopy (FESEM). In addition, the optical band gap of the sample was measured to be in the range from 2.410 eV to 2.697 eV after sintering temperature. From the data, it is believed that a cleaner production of low-cost zinc silicate can be achieved by using arecanut husk and have the potential to be used as phosphors materials.

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This paper proposes a novel idea to enhance the sensitivity and selectivity of surface plasmon resonance (SPR) optical sensor for detection of dengue virus type-2 envelope proteins (DENV-2 E-proteins) using polyamidoamine (PAMAM) dendrimer biopolymer-based nanocomposite thin film. For this purpose, two ranges of DENV-2 E-protein concentrations, i.e., 0.000008-0.0001 nM and 0.00008-0.005 nM were evaluated, and the lowest detectable concentration was achieved at 0.00008 nM. The incorporation of PAMAM dendrimer-based nanocomposite thin film with an SPR sensor exhibited a significant increase in sensitivity and binding affinity to a lower range DENV-2 E-protein concentrations. Moreover, the proposed sensor displayed good selectivity towards DENV-2 E-proteins and have an average recovery of 80-120%. The findings of this study demonstrated that PAMAM dendrimer-based nanocomposite thin film combined with SPR sensor is a promising diagnostic tool for sensitive and selective detection of DENV-2 E-proteins.

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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.

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In this study, synthesis and characterization of chitosan/maghemite (Cs/Fe2O3) 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/Fe2O3 thin film. The surface morphology of the thin film indicated the relatively smooth and homogenous thin film, and also confirmed the interaction of Fe2O3 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/Fe2O3 thin film with an optical-based method, i.e., surface plasmon resonance spectroscopy showed positive response where mercury ion (Hg2+) can be detected down to 0.01 ppm (49.9 nM). These results validate the potential of Cs/Fe2O3 thin film for optical sensing applications in Hg2+ detection.

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Great efforts have been devoted to the invention of environmental sensors as the amount of water pollution has increased in recent decades. Chitosan, cellulose and nanocrystalline cellulose are examples of biopolymers that have been intensively studied due to their potential applications, particularly as sensors. Furthermore, the rapid use of conducting polymer materials as a sensing layer in environmental monitoring has also been developed. Thus, the incorporation of biopolymer and conducting polymer materials with various methods has shown promising potential with sensitively and selectively toward heavy metal ions. In this feature paper, selected recent and updated investigations are reviewed on biopolymer and conducting polymer-based materials in sensors aimed at the detection of heavy metal ions by optical methods. This review intends to provide sufficient evidence of the potential of polymer-based materials as sensing layers, and future outlooks are considered in developing surface plasmon resonance as an excellent and valid sensor for heavy metal ion detection.

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In this work, waste coconut husk ash was used to prepare a ZnO-SiO2 composite. Solid-state technique was used to fabricate the composite due to its producibility, simple procedure as well as lower production cost. At high sintering temperatures ranging from 600 °C to 1000 °C, the X-ray diffraction (XRD) peaks of the Zn2SiO4 showed high intensity, which indicated high crystallinity. Furthermore, the formation of broad bands of ZnO4, Si-O-Si, and SiO4 were detected by Fourier transform infrared (FTIR) spectroscopy and the bands became narrower with the increment of sintering temperature. Besides, the morphological image from field emission scanning electron microscopy (FESEM) showed the formation of densely packed grains and smooth surface composite with the increase of sintering temperature. Upon obtaining the absorbance spectrum from Ultraviolet-Visible (UV-Vis) spectroscopy, the optical band gap was calculated to be 4.05 eV at 1000 °C. The correlation between the structural and optical properties of ZnO-SiO2 composite was discussed in detail.

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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) (Zn2+, Ni2+, and Fe3+). This technique generated significant results and showed that CTAB/HGQDs thin film has great potential for HMIs detection.

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Dengue viral infection is one of the most common deadliest diseases and has become a recurrent issue for public health in tropical countries. Although the spectrum of clinical diagnosis and treatment have recently been established, the efficient and rapid detection of dengue virus (DENV) during viremia and the early febrile phase is still a great challenge. In this study, a dithiobis (succinimidyl undecanoate, DSU)/amine-functionalized reduced graphene oxide--polyamidoamine dendrimer (DSU/amine-functionalized rGO-PAMAM) thin film-based surface plasmon resonance (SPR) sensor was developed for the detection of DENV 2 E-proteins. Different concentrations of DENV 2 E-proteins were successfully tested by the developed SPR sensor-based system. The performance of the developed sensor showed increased shift in the SPR angle, narrow full-width-half-maximum of the SPR curve, high detection accuracy, excellent figure of merit and signal-to-noise ratio, good sensitivity values in the range of 0.08-0.5 pM (S = 0.2576°/pM, R2 = 0.92), and a high equilibrium association constant (KA) of 7.6452 TM-1. The developed sensor also showed a sensitive and selective response towards DENV 2 E-proteins compared to DENV 1 E-proteins and ZIKV (Zika virus) E-proteins. Overall, it was concluded that the Au/DSU/amine-functionalized rGO-PAMAM thin film-based SPR sensor has potential to serve as a rapid clinical diagnostic tool for DENV infection.

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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/NH2rGO/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 NH2rGO-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 (R2 = 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 (R2 = 0.99). High selectivity of the SPR sensor towards DENV-2 E-proteins was achieved in the presence of other competitors.

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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 (Co2+) detection has been developed. The interaction of different Co2+ 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 Co2+. The sensitivity obtained was 0.1188 ppm-1 for low concentration of Co2+ ranged from 0.01 to 1 ppm. The FWHM and DA were consistent for all concentration of Co2+, while the SNR of the SPR signal increased with the Co2+ 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 Co2+ via SPR spectroscopy.

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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.

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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 (Co2+) 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 Co2+ interact with the graphene-based material sensor chip. The results demonstrated that the proposed sensor-based graphene layer was able to detect DENV and Co2+ 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.

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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 (Zn2+) detection that exists in the environment due to industrial processing. By replacing the gold thin film with a novel modified-gold thin film, Zn2+ can be detected from the range of 0 to 10 ppm using SPR. It is believed that the Zn2+ 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 Zn2+ as low as 0.01 ppm. The SPR results also followed the Langmuir isotherm model with a binding affinity of 1.927 × 103 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 Zn2+ compared to other metal ions, i.e. Ni2+, Fe2+, Cr2+, Mn2+, and Co2+.

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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 Ni2+ from 0.01 until 0.1 ppm with binding affinity constant of 1.620 × 103 M-1. The sensitivity for the CTA-NCC/GO calculated was 1.509°â€¯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 Ni2+ concentrations. For the selectivity test, the CTA-NCC/GO has the abilities to differentiate Ni2+ in the mixture of metal ions.

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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 Cu2+ 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 × 103 M-1. A high sensitivity of 3.271° ppm-1 also was obtained for low Cu2+ 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 Cu2+ 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 Cu2+, were 3.51 nm and 2.46 nm, respectively.