Carbon dots (CDs) possess unique optical properties such as tunable photoluminescence (PL) and excitation dependent multicolor emission. The quenching and recovery of the fluorescence of CDs can be utilized for detecting analytes. The PL mechanisms of CDs have been discussed in previous articles, but the quenching mechanisms of CDs have not been summarized so far. Quenching mechanisms include static quenching, dynamic quenching, Forster resonance energy transfer (FRET), photoinduced electron transfer (PET), surface energy transfer (SET), Dexter energy transfer (DET) and inner filter effect (IFE). Following an introduction, the review (with 88 refs.) first summarizes the various kinds of quenching mechanisms of CDs (including static quenching, dynamic quenching, FRET, PET and IFE), the principles of these quenching mechanisms, and the methods of distinguishing these quenching mechanisms. This is followed by an overview on applications of the various quenching mechanisms in detection and imaging.

The quenching of the fluorescence of carbon dots: A review on mechanisms and applications

There is great need for stand-alone luminescence-based chemosensors that exemplify selectivity, sensitivity, and applicability and that overcome the challenges that arise from complex, real-world media. Discussed herein are recent developments toward these goals in the field of supramolecular luminescent chemosensors, including macrocycles, polymers, and nanomaterials. Specific focus is placed on the development of new macrocycle hosts since 2010, coupled with considerations of the underlying principles of supramolecular chemistry as well as analytes of interest and common luminophores. State-of-the-art developments in the fields of polymer and nanomaterial sensors are also examined, and some remaining unsolved challenges in the area of chemosensors are discussed.

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Supramolecular Luminescent Sensors

Carbon quantum dots (CDs) are a relatively new class of carbon nanomaterials which have been studied very much in the last fifteen years to improve their already favorable properties. The optical properties of CDs have drawn particular interest as they display the unusual trait of excitation-dependent emission, as well as high fluorescence quantum yields (QY), long photoluminescence (PL) decay lifetimes, and photostability. These qualities naturally lead researchers to apply CDs in the field of imaging (particularly bio-imaging) and sensing. Since the amount of publications regarding CDs has been growing nearly exponentially in the last ten years, many improvements have been made in the optical properties of CDs such as QY and PL lifetime. However, a great deal of confusion remains regarding the PL mechanism of CDs as well as their structural properties. Therefore, presented in this review is a summary and discussion of the QYs and PL lifetimes reported in recent years. The effect of method as well as precursor has been evaluated and discussed appropriately. The current theories regarding the PL mechanism of CDs are discussed, with special attention to the concept of surface state-controlled PL. With this knowledge, the improvement of preparation and applications of CDs related to their optical properties will be easily accomplished. Further improvements can be made to CDs through the understanding of their structural and optical properties.

Recent development of carbon quantum dots regarding their optical properties, photoluminescence mechanism, and core structure

Schiff bases and their metal complexes have become well-known for catalytic (e.g., in various synthetic processes) and biological properties (e.g., antifungal, antibacterial, anti-malarial, and antiviral characteristics) since their discovery by Hugo Schiff in 1864. As synthetic compounds, they are employed as versatile tools in numerous applications such as fluorescent turn-on/turn-off sensors for the determination of diverse analytes (e.g., metallic components). As such, they can offer a way to identify toxic ions and/or to provide their speciation in environmental media. This review covers a broad range of Schiff bases that are used in sensing applications for metallic cations and anions in various kinds of environmental and biological media.

A review of the applications of Schiff bases as optical chemical sensors

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

Fluorescein derivatives are important fluorescent probes which can be used for detection and optical imaging. Fluorescein derivatives are usually constructed by introducing aldehyde groups or esterified onto fluorescein xanthene ring and benzene moiety. Typically, the research direction of connecting amino groups with fluorescein monoaldehyde is in hot. Because of their high activity, these derivatives can be complexed with the analytes to produce changes of colors and the increase or decrease of fluorescence intensity. This article reviewed fluorescein probes in the past two years according to classification of different analytes including metal ions, anions, small molecules and biological macromolecules. The synthesis methods, optical properties, possible mechanisms and applications of fluorescein probes are summarized. This article provides a reference for the screening of fluorescein probes with high sensitivity and biological detection and can propel their further applications in sensing and detection of analytes.

Fluorescein applications as fluorescent probes for the detection of analytes

Ratiometric fluorescent probes allow the simultaneous measurement of two fluorescence signals at different wavelengths followed by calculation of their intensity ratio, which can provide more precise measurement results than intensity-based fluorescent probes Excitation energy transfer is widely used in the design of ratiometric fluorescent probes Rhodamine is a convenient platform for the construction of “OFF–ON” ratiometric chemosensors Rhodamine-based ratiometric fluorescent probes based on the excitation energy transfer mechanism can be constructed by conjugated or non-conjugated connections with other chromophores In this review, we summarized the recent advances regarding rhodamine-based ratiometric fluorescent probes based on excitation energy transfer We reviewed these probes according to the classification of “through-space” and “through-bond” probes; we focused on the contributions of different donor fluorophores and the types of connections between the energy donors and acceptors

Rhodamine-based ratiometric fluorescent probes based on excitation energy transfer mechanisms: construction and applications in ratiometric sensing

The CDs were used as the ratiometric probe for detection of curcumin and Fe3+. The fluorescence of CDs at 420 nm was quenched through inner filter effect (IFE) while the fluorescence of curcumin at 525 nm was enhanced after adding curcumin. The value of ln(I525/I420) increased linearly with the curcumin concentration in the range of 5–30 μM, the limit of detection was 21.79 nM. After adding Fe3+ into the system of CDs-curcumin, curcumin complexed with Fe3+ leading to the quenching of the fluorescence of curcumin at 525 nm, which declined the spectra overlap between curcumin and CDs. The process of IFE was inhibited, the fluorescence of CDs at 420 nm was recovered. The value of ln(I525/I420) linearly decreased with the increasing of Fe3+ in the range of 2-14 μM, the limit of detection is 18.11 nM. The CDs-curcumin system was successfully applied for H1299 cell imaging and has potential applications in the fields of fluorescent labeling and inhibition of tumor cells. The CDs-curcumin system was furtherly used for PVDF membrane fouling research of iron flocculants in wastewater treatment.

Fluorescent carbon dots for ratiometric detection of curcumin and ferric ion based on inner filter effect, cell imaging and PVDF membrane fouling research of iron flocculants in wastewater treatment

A novel FRET ratiometric fluorescent probe (CMH-GA-CDs) was designed for the detection of Cu2+ in aqueous solution. The CMH-GA-CDs were prepared through grafting 7-diethylaminocoumarin-3-carbohydrazide (HCM) on the surface of glyoxylic acid-modified carbon dots (GA-CDs) by amidation reaction. In the absence of Cu2+, a FRET pair from the CDs to CMH units was established, in which the CMH-GA-CDs exhibited dual emission bands centered at 400 nm (CDs) and 458 nm(CMH), respectively, under optimal excitation wavelength of CDs (λex = 340 nm). Upon addition of 30 μM Cu2+ to CMH-GA-CDs solution, Cu2+ coordinated with the heteroatoms N and O, which caused the inhibition of FRET process and the change of fluorescence intensity ratio I458/I400 from 1.95 to 0.91. The ratio of I458/I400 exhibited a good linear relationship with the concentration of Cu2+ in the range of 0–10 μM and the limit of detection was as low as 0.21 μM. Furthermore, The CMH-GA-CDs were used for the detection of Cu2+ in tap water and lake water with satisfactory recovery ratio between 91% and 104%, demonstrating the promising applicability for detecting Cu2+ in water samples.

Jinxia Xu

Papers

The quenching of the fluorescence of carbon dots: A review on mechanisms and applications

Fluorescein applications as fluorescent probes for the detection of analytes

Rhodamine-based ratiometric fluorescent probes based on excitation energy transfer mechanisms: construction and applications in ratiometric sensing

Fluorescent carbon dots for ratiometric detection of curcumin and ferric ion based on inner filter effect, cell imaging and PVDF membrane fouling research of iron flocculants in wastewater treatment

Ratiometric fluorescent detection of copper ions using coumarin-functionalized carbon dots based on FRET