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 (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.

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.

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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.

Surface plasmon resonance (SPR) spectroscopy has emerged as an optical sensor for sensing a variety of analytes, including metal ions. However, despite its numerous advantages, which include very high sensitivity, simple sample preparation, low cost, fast measurement capability, no requirement for reference solution, high reproducibility, ability to monitor kinetic behaviour, label-free detection, and nondestructiveness, the SPR optical sensor has to compete with existing methods especially in terms of sensitivity and selective detection. A critical review of the use of SPR in metal ion detection is presented. It describes the instrument and different developments on active layers or recognition molecules for sensitivity and selectivity improvements. In conclusion, progress in SPR optical sensor technology will further expand SPR detection abilities and allow SPR sensing to be used widely including in environmental monitoring as an effective metal ion sensor in the future.

The plethora of optical techniques have been developed to detect the dengue virus, regrettably, the existing techniques are cumbersome and have limitations to provide early identification of the lowlevel of dengue virus. For this purpose, surface plasmon resonance (SPR) optical sensor was designed with a highly sensitive thin film based on hyperbranched polymer nanocomposites for conjugation of antibodies that specifically detect the dengue virus type II (DENV). In particular, we investigate the SPR sensing performance for gold/cadmium sulfide quantum dotspolyamidoamine/antibody (Au/CdS QDs-PAMAM/IgM) sensor film in terms of linear sensitivity, binding affinity, detection accuracy, full-width-half-maximum, selectivity, and spike and recovery test. The results show a strong affinity value, good sensitivity and selectivity, high accuracy, and excellent recovery value, obeying the ideal value based on conventional assay analysis. Additionally, this Au/CdS QDs-PAMAM/IgM based SPR sensor was successfully applied for the lowest detection of 1 pM of DENV E-proteins. To this end, a highly sensitive Au/CdS QDs-PAMAM/IgM sensor film was characterized using Raman spectroscopy, ultraviolet-visible (UV-Vis-NIR) spectroscopy, and atomic force microscopy (AFM) to confirm the binding of DENV.

In this research, the preparation of chitosan and hydroxyl-functionalized graphene quantum dots (HGQDs) composite has been described. The spin coating technique was used to prepare the chitosan/hydroxyl-functionalized graphene quantum dots (Cs/HGQDs) thin film. The Cs/HGQDs thin film was then characterized using the Fourier transform infrared spectroscopy which confirmed the existence of amino groups, carboxylic acid groups, carboxyl groups and hydroxyl groups in Cs/HGQDs. UV-Vis absorption spectroscopy and photoluminescence (PL) were used to study the optical properties of the thin film. The absorption of Cs/HGQDs thin film was high with optical band gap of 3.797 eV. The intensity of PL spectra of the thin film was observed around wavelength of 420 nm. The incorporation of Cs/HGQDs thin film with surface plasmon resonance spectroscopy produced positive responses towards the Fe³⁺ ion solutions of different concentration and it was found that Cs/HGQDs thin film able to detect Fe³⁺ as low as 0.5 ppm with a sensitivity of 0.11396° ppm⁻¹. Subsequently, Cs/HGQDs layer have high potential as sensing layer to detect Fe³⁺ due to high affinity of Fe³⁺ ion towards the thin film with a value of binding affinity constant, K equals to 5.79 ppm⁻¹. Atomic force microscopy was used to observe the surface morphology of the thin film and the result indicates that the thin film is relatively smooth and homogenous which also confirmed the interaction of Fe³⁺ with the thin film. Thus, Cs/HGQDs thin film shows potential for the detection of Fe³⁺ in solution.