Carbon dots (CDs) display tunable photoluminescence and excitation-wavelength dependent emission. The color of fluorescence is affected by electronic bandgap transitions of conjugated π-domains, surface defect states, local fluorophores and element doping. In this review (with 145 refs.), the studies performed in the past 5 years on the relationship between the fluorescence mechanism and modes for modulating the emission color of CDs are summarized. The applications of such CDs in sensors and assays are then outlined. A concluding section then gives an outlook and describes current challenges in the design of CDs with different emission colors. Graphical abstract Schematic representation of the relationship between the color-emitting (blue, green, yellow, red and multicolor) modulation of carbon dots and fluorescence mechanism including bandgap transitions of conjugated π-domains and surface defect states.

The fluorescence mechanism of carbon dots, and methods for tuning their emission color: a review

Due to the risks that water contamination implies for human health and environmental protection, monitoring the quality of water is a major concern of the present era. Therefore, in recent years several efforts have been dedicated to the development of fast, sensitive, and selective sensors for the detection of heavy metal ions. In particular, fluorescent sensors have gained in popularity due to their interesting features, such as high specificity, sensitivity, and reversibility. Thus, this review is devoted to the recent advances in fluorescent sensors for the monitoring of these contaminants, and special focus is placed on those devices based on fluorescent aptasensors, quantum dots, and organic dyes.

https://www.mdpi.com/1424-8220/19/3/599/pdf

Fluorescent Sensors for the Detection of Heavy Metal Ions in Aqueous Media.

Metal halide perovskites have been intensively studied over the past years due to their attractive electronic, optical and optoelectronic properties In spite of the phenomenally high solar-conversion efficiency achieved with perovskites, their adverse instability remains the key obstacle restricting their wider applications These ionic crystals can be affected by many factors, such as varied temperatures, humid air, polar solvents, and electron-withdrawing/donating gases However, under certain controlled conditions, changes in perovskite structures/compositions via, for example, phase transitions, hydration/dehydration, gas adsorption/desorption, and ion intercalation/decalation, can all be reversed, suggesting their great potential for sensing applications In this contribution, we first overview recent investigations and mechanistic studies on the stability of metal halide perovskites under various environmental conditions Then, we highlight the recent attempts to apply perovskites for the detection of temperature change, humidity, gases, solvents and ions Finally, we discuss current challenges and future opportunities for developing sensing systems based on metal halide perovskites

Metal halide perovskites: stability and sensing-ability

Research related to the development and application of luminescent nanoparticles (LNPs) for chemical and biological analysis and imaging is flourishing. Novel materials and new applications continue to be reported after two decades of research. This review provides a comprehensive and heuristic overview of this field. It is targeted to both newcomers and experts who are interested in a critical assessment of LNP materials, their properties, strengths and weaknesses, and prospective applications. Numerous LNP materials are cataloged by fundamental descriptions of their chemical identities and physical morphology, quantitative photoluminescence (PL) properties, PL mechanisms, and surface chemistry. These materials include various semiconductor quantum dots, carbon nanotubes, graphene derivatives, carbon dots, nanodiamonds, luminescent metal nanoclusters, lanthanide-doped upconversion nanoparticles and downshifting nanoparticles, triplet-triplet annihilation nanoparticles, persistent-luminescence nanoparticles, conjugated polymer nanoparticles and semiconducting polymer dots, multi-nanoparticle assemblies, and doped and labeled nanoparticles, including but not limited to those based on polymers and silica. As an exercise in the critical assessment of LNP properties, these materials are ranked by several application-related functional criteria. Additional sections highlight recent examples of advances in chemical and biological analysis, point-of-care diagnostics, and cellular, tissue, and in vivo imaging and theranostics. These examples are drawn from the recent literature and organized by both LNP material and the particular properties that are leveraged to an advantage. Finally, a perspective on what comes next for the field is offered.

Photoluminescent Nanoparticles for Chemical and Biological Analysis and Imaging

Recently, perovskite-based nanomaterials are utilized in diverse sustainable applications. Their unique structural characteristics allow researchers to explore functionalities towards diverse directions, such as solar cells, light emitting devices, transistors, sensors, etc. Many perovskite nanomaterial-based devices have been demonstrated with extraordinary sensing performance to various chemical and biological species in both solid and solution states. In particular, perovskite nanomaterials are capable of detecting small molecules such as O2, NO2, CO2, etc. This review elaborates the sensing applications of those perovskite materials with diverse cations, dopants and composites. Moreover, the underlying mechanisms and electron transport properties, which are important for understanding those sensor performances, will be discussed. Their synthetic tactics, structural information, modifications and real time sensing applications are provided to promote such perovskite nanomaterials-based molecular designs. Lastly, we summarize the perspectives and provide feasible guidelines for future developing of novel perovskite nanostructure-based chemo- and biosensors with real time demonstration.

https://www.mdpi.com/2227-9040/8/3/55/pdf

Review on Sensing Applications of Perovskite Nanomaterials

Pollution triggered by highly toxic heavy metal ions has become a worldwide issue of concern, and has aroused increasing research interest, but it is still a challenge to carry out its trace detection in the organic phase using inorganic fluorescent colloidal nanocrystals (NCs). Herein, we report fully inorganic CsPbBr3 perovskite quantum dots (PQDs), which were synthesized via a hot-injection method, as a fluorescent probe for the selective detection of copper ions in hexane. The photoluminescence (PL) intensity of CsPbBr3 PQDs is significantly quenched within several seconds after the addition of Cu2+ due to effective electron transfer from the PQDs to the added Cu2+, which is experimentally verified via analyzing absorption spectra and PL decay lifetime. The sensor, in turn-off mode, has a detection range from 0 to 100 nM with a limit of detection (LOD) as low as 0.1 nM, indicating great potential for practical applications. This study paves the way for PQD-based fluorescent probes for heavy metal detection in the organic phase.

All-inorganic CsPbBr3 perovskite quantum dots as a photoluminescent probe for ultrasensitive Cu2+ detection

A highly selective fluorescent probe for Hg2+ is reported. It consists of nitrogen doped graphene quantum dots (NGQDs) that are nearly spherical in shape, have an average diameter of 2.7 nm and excitation-independent emission. The blue fluorescence of the NGQDs (with maximum excitation/emission at 378/447 nm) is quenched by Hg2+ due to both dynamic and static quenching. The probe has a wide detection range (2.5 μM – 800 μM) and a limit of detection of 2.5 μM. The dynamic and static quenching constants are 417 M−1 and 63500 M−1, respectively. The probe was used to quantfy Hg2+ in spiked real water samples with satisfactory results.

Nitrogen doped graphene quantum dots as a fluorescent probe for mercury(II) ions

Long range phase-sensitive optical time domain reflectometer (ϕ-OTDR) sensing mainly employs distributed amplification in the sensing fiber. It requires the light to be injected into both ends of the sensing fiber, which reduces the degree of freedom in embedding the fiber into structures. To overcome this problem, the key factors that affects the signal-to-noise ratio (SNR) in ϕ-OTDR system based on matched filter is analyzed thoroughly, and a single-ended long-range ϕ-OTDR that does not require distributed amplification is proposed. In this system, two key techniques are adopted for SNR improvement. To boost the pulse energy and suppress the self-phase modulation, the distortion of the amplified pulse is rectified by using iterative predistortion method; to mitigate the influence of the interference fading and the stimulated Brillouin backscattering, a three-carrier pulse is employed. In combination with the non-linear frequency modulation technique which yields a 42.7 dB side lobe suppression ratio, these approaches guarantee an achievable ϕ-OTDR of 80 km sensing range, 2.7 m spatial resolution, 49.6 dB dynamic range in the experiment. To the best of authors’ knowledge, this is the first time that a ϕ-OTDR without optical amplification in the sensing fiber is realized over such a long sensing range.

80 km Fading Free Phase-Sensitive Reflectometry Based on Multi-Carrier NLFM Pulse Without Distributed Amplification

Selective and sensitive detection of copper(II) based on fluorescent zinc-doped AgInS2 quantum dots

The round-trip time of the light pulse limits the maximum detectable vibration frequency response range of phase-sensitive optical time domain reflectometry (ϕ-OTDR). Unlike the uniform laser pulse interval in conventional ϕ-OTDR, we randomly modulate the pulse interval so that an equivalent sub-Nyquist additive random sampling (sNARS) is realized for every sensing point of the long interrogation fiber. For a ϕ-OTDR system with 10 km sensing length, the sNARS method is optimized by theoretical analysis and Monte Carlo simulation, and the experimental results verify that a wideband sparse signal can be identified and reconstructed. Such a method can broaden the vibration frequency response range of ϕ-OTDR, which is of great significance in sparse-wideband-frequency vibration signal detection.

Yongzhong Bai

Papers

All-inorganic CsPbBr3 perovskite quantum dots as a photoluminescent probe for ultrasensitive Cu2+ detection

Nitrogen doped graphene quantum dots as a fluorescent probe for mercury(II) ions

80 km Fading Free Phase-Sensitive Reflectometry Based on Multi-Carrier NLFM Pulse Without Distributed Amplification

Selective and sensitive detection of copper(II) based on fluorescent zinc-doped AgInS2 quantum dots

Distributed fiber sparse-wideband vibration sensing by sub-Nyquist additive random sampling.