This article discusses three aspects of inner filter effect (IFE) in fluorescence spectroscopy. (i) First, IFE as undesirable in fluorescence measurements: IFE results in non-linear fluorescence response of the analyte under study and it has been verified that IFE cannot be eliminated; it can either be minimized or corrected for intensity loss. Over the years, researchers have proposed many intensity correction methods to avoid IFE related issues. Often analysts using fluorescence spectroscopy, knowingly or unknowingly, ignore IFE or use an inappropriate intensity correction method. Herein, we have highlighted the basis and significances of various correction models that are proposed since 1970s to till date. (ii) Second, IFE mediated concentration dependent red shift (CDRS) as an analytical tool: the conventional fluorescence measurements and IFE correction strategies cannot be applied in analysis of optically dense multi-fluorophoric samples like oils, petrochemicals, biological samples and food samples etc. The strategy for exclusive inclusion of IFE and IFE induced CDRS as characteristics of the system for development of fluorescence based assay, towards maximizing fluorescence sensitivity of optically dense multi-fluorophoric systems, have been discussed. (iii) Third, IFE based sensing: when the sample contains chromophores, which absorb either at the excitation or at the emission wavelength range of the fluorophore, then the chromophores act as a filter. Thus tuning either the absorber or fluorophore concentration will lead to development of fluorescence based assay for a selective analyte. Principles and protocols are described to identify whether a sensing event is due to IFE or any other fluorescence mechanism. Additionally, a brief description is given on advanced findings and progresses made in sensing of for various classes of analytes in the recent past, using IFE concept. The second and third aspect combined together serve as a tool towards enhancing sensitivity of fluorescence measurement.

Inner filter effect in fluorescence spectroscopy: As a problem and as a solution

Fluorescence carbon nanomaterials-based sensors for accurate tracing of pesticides in environmental, food and biological samples have gained considerable interest in the past decade years to protect ecosystem, guarantee food safety and prevent disease, making it a trending hot-spot. This review focuses on recent advances and new trends of pesticides monitoring (including organophosphorus, neonicotinoids and carbamates pesticides) on the base of fluorescence carbon nanomaterials, with particular emphasis on zero-dimensional (0-D) carbon dots/graphene quantum dots, 2-D carbon nitride nanosheets and 3-D carbon-based metal/covalent organic frameworks. These fluorometric strategies are classified by several main recognition unit, including enzyme, antibody, aptamer and molecularly-imprinted polymers, offering great performance for pesticides with a limit of detection down to pg mL−1 level. Beyond performance comparison of these emerging sensors with other strategies, we discuss the existing hurdles and present challenges to practical sensor designs, followed by exploring future perspectives for breakthroughs in fluorescence carbon nanomaterials-based sensors.

Biosensors based on fluorescence carbon nanomaterials for detection of pesticides

Advances and perspectives in carbon dot-based fluorescent probes: Mechanism, and application

Charge storage in binder-free 2D-hexagonal CoMoO4 nanosheets as a redox active material for pseudocapacitors

4-nitrophenol optical sensing with N doped oxidized carbon dots

Nanoceria with a remarkable phosphatase mimicking activity was synthesized and used to catalyze the hydrolysis of phosphate esters in pH 10 solution. The catalytic effect of nanoceria was firstly investigated by selecting p-nitrophenyl phosphate as a model substrate. The pH value, incubation temperature, reaction time, and concentration of nanoceria were optimized. The catalytic effect was then confirmed by using methyl-paraoxon as a substrate. The p-nitrophenol anion released by the enzyme mimic is yellow and exerts an inner filter effect on the fluorescence of the carbon dots (with excitation/emission maxima at 400/520 nm). Response to methyl-paraoxon is linear in the 1.125–26.25 μmol L−1 concentration range. The method was applied to the determination of pesticides in spiked Panax quinquefolius and water samples. Recoveries ranged from 85 to 103% (n = 3). The technique is rapid, reliable, and can be used for on-site detection of pesticides and organophosphates.

Fluorometric determination of pesticides and organophosphates using nanoceria as a phosphatase mimic and an inner filter effect on carbon nanodots

Colloidal all-inorganic perovskites nanocrystals (NCs) have emerged as a promising material for display and lighting due to their excellent optical properties. However, blue emissive NCs usually suffer from low photoluminescence quantum yields (PLQYs) and poor stability, rendering them the bottleneck for full-color all-perovskite optoelectronic applications. Herein, a facile approach is reported to enhance the emission efficiency and stability of blue emissive perovskite nano-structures via surface passivation with potassium bromide. By adding potassium oleate and excess PbBr2 to the perovskite precursor solutions, potassium bromide-passivated (KBr-passivated) blue-emitting (≈450 nm) CsPbBr3 nanoplatelets (NPLs) is successfully synthesized with a respectably high PLQY of 87%. In sharp contrast to most reported perovskite NPLs, no shifting in emission wavelength is observed in these passivated NPLs even after prolonged exposures to intense irradiations and elevated temperature, clearly revealing their excellent photo- and thermal-stabilities. The enhancements are attributed to the formation of K-Br bonding on the surface which suppresses ion migration and formation of Br-vacancies, thus improving both the PL emission and stability of CsPbBr3 NPLs. Furthermore, all-perovskite white light-emitting diodes (WLEDs) are successfully constructed, suggesting that the proposed KBr-passivated strategy can promote the development of the perovskite family for a wider range of optoelectronic applications.

Stable and Efficient Blue-Emitting CsPbBr3 Nanoplatelets with Potassium Bromide Surface Passivation.

Quantum dot light emitting diodes (QLEDs) have been emerging with unique photoelectrical properties and their efficiency is rapidly approaching the commercialization requirement. However, the device operation lifetime is still one of the biggest obstacles facing the QLED applications. Here, we investigate the case of degradation of red QLEDs by a low driving level (7.5 mA cm−2) because devices can commonly achieve a luminance of ∼500–2000 cd m−2 (that is required in display applications) under this driving current. Based on the photoelectrical characteristics of QLEDs with different aging times, we find that the device stability is dominantly affected by the interfaces between QDs and adjacent charge transport layers. Some of the QDs stay in the permanent dark states (lightless states) due to the aging process, which may result from the generation of nonradiative sites in the hole-transport layer (HTL). The electron leakage should be responsible for the degradation of the HTL, leading to a highly resistive HTL and the rise of the operation voltage for the aged devices. Therefore, suppressing the interactions between QDs and charge transport layers and confining the charges within the QDs are feasible ways to boost the operation stability of QLEDs.

Degradation of quantum dot light emitting diodes, the case under a low driving level

As a sustainable and clean water production technology, solar thermal water evaporation has been extensively studied in the past few years. One challenge is that upon operation, salt would form on surface of the solar absorbers leading to inefficient water supply and light absorption and thus much reduced water vaporization rate. To address this problem, a simple solar evaporator based on an array of aligned millineedles for efficient solar water evaporation and controlled site-specific salt formation is demonstrated. The maximum solar evaporation rate achieved is 2.94 kg m-2 h-1 under one Sun irradiation in brine of high salinity (25 wt% NaCl), achieving energy conversion efficiency of 94.5% simultaneously. More importantly, the spontaneously site-specific salt formation on the tips of millineedles endows this solar evaporator with salt harvesting capacity. Rationally separating the clean water and salt from brine by condensation and gravity assistance, this tip-preferential crystallization solar evaporator is not affected by the salt clogging compared with conventional 2D solar evaporators. This study provides new insights on the design of solar evaporators and advances their applications in sustainable seawater desalination and wastewater management.

Aligned Millineedle Arrays for Solar Power Seawater Desalination with Site-Specific Salt Formation.

Chinese herbs have been widely used for thousands of years. In order to kill or control insects and fungus during the cultivation of herb plants, pesticides have played a significant role. More than 30 kinds of pesticides have been documented in the latest version of Chinese Pharmacopoeia. It is urgent to develop new analytical methods for pesticide detection. A fluorescent detection system was established by using Cerium based fluorescent polymer and Sm-CeO2. As a new doped nanozyme, Sm-CeO2 exhibits OPH-like activity to hydrolyze OPP pesticide. The morphology of the prepared CFP and Sm-CeO2 were characterized. The optimal conditions for CFP synthesis are CeCl3 (16 mmol L−1, 200 μL), ATP (4 mmol L−1, 200 μL) and Tris buffer (5 mmol L−1, 500 μL, pH 8.0). Sm-CeO2 shows the best mimic activity to hydrolyze OPP pesticide at pH = 8.0. The results exhibit good linear relationship between fluorescent quenching effect and MP in the range of 2–50 μmol L−1. Finally, this fluorescent technique was successfully applied in Poria cocos and Semen Coicis sample. It is the first report on OPP detection by using CFP and doped nanozyme. The successful application in real sample indicates this method is a rapid, reliable strategy to detect OPP in Chinese herbs.

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Jia-Yi Dong

Papers

Fluorometric determination of pesticides and organophosphates using nanoceria as a phosphatase mimic and an inner filter effect on carbon nanodots

Stable and Efficient Blue-Emitting CsPbBr3 Nanoplatelets with Potassium Bromide Surface Passivation.

Degradation of quantum dot light emitting diodes, the case under a low driving level

Aligned Millineedle Arrays for Solar Power Seawater Desalination with Site-Specific Salt Formation.

A new fluorescent technique for pesticide detection by using metal coordination polymer and nanozyme