Recently, synthesis, characterization, and application of carbon dots have received much attention. Natural products are the effectual carbon precursors to synthesize carbon dots with fascinating chemical and physical properties. In this study, the fluorescent sensor of carbon dots derived from cranberry beans without any functionalization and modification was developed. The carbon dots were prepared with a cheap, facile, and green carbon precursor through a hydrothermal treatment method. The synthetic process was toxic chemical-free, convenient, and environmentally friendly. To find the optimized synthetic conditions, the temperature, heating time duration, and carbon precursor weight were evaluated. The prepared carbon dots were characterized by UV light, transmission electron microscopy, Raman, Fourier transform infrared, UV–vis, and fluorescence spectroscopy. The resulting carbon dots exhibit stable fluorescence with a quantum yield of approximately 10.85%. The carbon dots emitted the broad fluorescence emission range between 410 and 540 nm by changing the excitation wavelength and were used for the detection of Fe³⁺ ions at the excitation of 380 nm. It is found that Fe³⁺ ions induced the fluorescence intensity quenching of the carbon dots stronger than other heavy metals and the Fe³⁺ ion detection can be achieved within 3 min. Spectroscopic data showed that the obtained carbon dots can detect Fe³⁺ ions within the wide concentration range of 30–600 μM with 9.55 μM detection limit.

Carbon dots (CDs) are a new cluster of carbon atoms with particle size less than 10 nm. CDs also exhibit interesting fluorescence (FL) properties. CDs are attractive because of their fascinating characteristics including low toxicity, good water solubility, and tremendous biocompatibility. Recently, CDs have been investigated as biosensors for numerous target analytes. Meanwhile, the utilization of cheap and renewable natural resources not only fulfills the pressing requirement for the large-scale synthesis of CDs but also encourages the establishment of sustainable applications. The preparation of CDs using natural resources, i.e., plants, offers several advantages as it is inexpensive, eco-friendly, and highly available in the surroundings. Plant parts are readily available natural resources as the starting materials to produce CDs with different characteristics and attractive applications. Several review articles are now available covering the synthesis, properties, and applications of CDs. However, there is no specific and focused review literature discussing plant part-derived CDs for biosensing applications. To handle this gap, we provide a review of the progress of CDs derived from various plant parts with their synthesis methods, optical properties, and biosensing applications in the last five years. We highlight the synthesis methods and then give an overview of their optical properties and applications as biosensors for various biomolecules and molecules in biological samples. Finally, we discuss some future perspectives for plant part-derived CDs for better material development and applications.

Carbon dots (CDs) have generated much interest because of their significant fluorescence (FL) properties, extraordinary photophysical attributes, and long-term colloidal stability. CDs have been regarded as a prospective carbon nanomaterial for various sensing applications because of their low toxicity, strong and broad optical absorption, high chemical stability, rapid transfer properties, and easy modification. To improve their functionality, CD/polymer composites have been developed by integrating polymers into CDs. CD/polymer composites have diversified because of their easy preparation and applications in sensing, optoelectronics, semiconductors, molecular delivery, and various commercial fields. Many review articles are available regarding the preparation and applications of CDs. Some review articles describing the production and multiple applications of the composites are available. However, no such article has focused on the types of precursors, optical properties, coating characteristics, and specific sensing applications of CD/polymer composites. This review aimed to highlight and summarize the current progress of CD/polymer composites in the last five years (2017–2021). First, we overview the precursors used for deriving CDs and CD/polymer composites, synthesis methods for preparing CDs and CD/polymer composites, and the optical properties (absorbance, FL, emission color, and quantum yield) and coating characteristics of the composites. Most carbon and polymer precursors were dominated by synthetic precursors, with citric acid and polyvinyl alcohol widely utilized as carbon and polymer precursors, respectively. Hydrothermal treatment for CDs and interfacial polymerization for CDs/polymers were frequently performed. The optical properties of CDs and CD/polymer composites were almost identical, denoting that the optical characters of CDs were well-maintained in the composites. Then, the chemical, biological, and physical sensing applications of CD/polymer composites are categorized and discussed. The CD/polymer composites showed good performance as chemical, biological, and physical sensors for numerous targets based on FL quenching efficiency. Finally, remaining challenges and future perspectives for CD/polymer composites are provided.

In this study, a simple and green hydrothermal treatment was performed to prepare nitrogen-doped carbon dots (NCDs) from Averrhoa carambola (AC) fruit extract as a carbon precursor and L-arginine (Arg) as a nitrogen dopant. The AC-NCDs were characterized by UV light, fluorescence spectroscopy, transmission electron microscopy, FTIR spectroscopy, Raman spectroscopy, UV-vis spectroscopy, and zeta potential analyzer. The AC-NCDs were spherical and the average diameter was estimated to be 6.67 nm. The AC-NCDs exhibited the maximum emission intensity at 446 nm with 360 nm excitation wavelength. The fluorescence quenching behavior of AC-NCDs after interacting with methyl orange (MO) dye was studied. The interaction of AC-NCDs and MO was achieved within 3 min and the fluorescence quenching was maintained to a fixed value even after 30 min. The linearity was obtained in the range of 1 to 25 μM MO with a 0.30 μM detection limit. Furthermore, the pH values affected the quenching behavior of the AC-NCDs/MO system where the interaction mechanisms were driven by the electrostatic interaction, π–π interaction, inner filter effect, and energy transfer. The pH 5 maintained higher quenching efficiency while other pH values slightly decreased the quenching efficiency. Incoming applications, the AC-NCDs can be used in various important fields, especially for environmental protection.

In this study, Volvariella volvacea (VV) mushroom was utilized as a green carbon precursor to synthesize carbon dots (CDs) and applied as a fluorescent (FL) sensor to detect Fe³⁺ and Pb²⁺ ions. The VV-CDs showed a high photostability and FL properties with a quantum yield of about 11.5%. The VV-CDs exhibited an excitation-dependent emission manner, with an optimum emission wavelength at 440 nm excited at 360 nm. The VV-CDs were evaluated and shown to be facile and effective FL sensors for detecting Pb²⁺ and Fe³⁺ ions based on the FL quenching efficiency, demonstrating the special complex chelate between the metal ions and effective surface functional groups of VV-CDs. VV-CDs displayed high responses to Fe³⁺ and Pb²⁺ ions, with the linear relationship of 1-100 μM and limits of detection (LODs) of 12 and 16 nM for Pb²⁺ and Fe³⁺ ions, respectively. Furthermore, the FL sensor was validated with real water samples, showing very good spike recoveries.

Contamination of nitroarenes (NAs) seriously endangers the environment and the health of organisms. Therefore, the development of a fluorescent sensor with good response for sensing NAs is crucial. Herein, eco-friendly carbon dots (CDs) from oyster mushroom (OM) as a sole carbon precursor were effectively produced via a facile hydrothermal treatment method. The properties of the OM-CDs were evaluated via TEM, FT-IR spectroscopy, Raman spectroscopy, UV-light, fluorescence spectroscopy, and UV-Vis spectroscopy. The OM-CDs possessed mono-dispersed spherical forms with a narrow size distribution. The OM-CDs also exhibited a cyan color excited by a 360 nm excitation wavelength with a 12.51% quantum yield. The fluorescent behavior of the OM-CDs was found to be dependent on the excitation wavelength, and they were used as a rapid and sensitive fluorescent sensor for NAs with broad linear responses and low detection limits. The NAs quenched the fluorescence emission intensity of the OM-CDs because of the inner filter effect (IFE), fluorescence resonance energy transfer (FRET), and photo-induced electron transfer (PET), which conferred the sensing mechanisms. The absorbance spectra of o-NA and p-NA have a wider overlap with the fluorescence excitation and emission spectra of OM-CDs than those of m-NA and p-NP, resulting in higher quenching efficiency of o-NA and p-NA. Eventually, this fluorescent sensor presented practicability to determine NAs in real water samples. The successful determinations of these NAs in the underground water samples were performed with RSDs below 2% and spiked recoveries between 97.20% and 102.60%.