Nanomaterial-Based Dual-Emission Ratiometric Fluorescent Sensors for Biosensing and Cell Imaging

Development of a Pyrone-Fused Tricyclic Scaffold-based Ratiometric Fluorescent Probe for Al3+ Detection

Aluminum (Al3+) is environmentally abundant and can harm living organisms in various ways, such as by inhibiting root growth, damaging faunal nervous systems, and promoting tumor cell proliferation. However, the dynamics of Al3+ in living organisms are largely unknown; thus, detecting Al3+ in the environment and organisms is crucial. Fluorescent probes are useful tools for the selective detection of metal ions. In particular, ratiometric fluorescent probes exhibit a detection response at two different maximum fluorescence emission wavelengths; which is advantageous for avoiding the influence of background fluorescence. A novel pyrone-fused tricyclic scaffold-based ratiometric fluorescent probe for detecting Al3+, ethyl 11-imino-1-oxo-3-phenyl-1H,11H-pyrano[4,3-b] quinolizine-5-carboxylate (PQ), was developed in this study. The PQ fluorescence blue shifted from 505 to 457 nm upon the addition of Al3+. The blue shift was accompanied by a change in the fluorescence color of the PQ solution from green to blue. Fluorescence titration experiments demonstrated that the fluorescence intensity ratio at the two peaks of interest (457/505 nm) increased in a concentration-dependent manner upon the addition of Al3+. Moreover, this study demonstrated that a PQ-soaked paper displays a visible color change under ultraviolet light upon exposure to Al3+. The above results suggest that PQ is an effective ratiometric probe for the detection of Al3+ in the environment. Future studies will be conducted to introduce various substituents and develop fluorescent probes by leveraging the fluorescence property of a pyrone-fused tricyclic scaffolds.

Luminescence Probes in Bio-Applications: From Principle to Practice

Bioanalysis based on optical imaging has gained significant progress in the last few decades. Luminescence probes are capable of detecting, monitoring, and tracing particular biomolecules in complex biological systems to figure out the roles of these molecules in organisms. Considering the rapid development of luminescence probes for bio-applications and their promising future, we have attempted to explore the working principles and recent advances in bio-applications of luminescence probes, in the hope of helping readers gain a detailed understanding of luminescence probes developed in recent years. In this review, we first focus on the current widely used luminescence probes, including fluorescence probes, bioluminescence probes, chemiluminescence probes, afterglow probes, photoacoustic probes, and Cerenkov luminescence probes. The working principles for each type of luminescence probe are concisely described and the bio-application of the luminescence probes is summarized by category, including metal ions detection, secretion detection, imaging, and therapy.

Ratiometric fluorescent and electrochemiluminescent dual modal assay for detection of 2,6-pyridinedicarboxylic acid as an anthrax biomarker

2,6-pyridinedicarboxylic acid (DPA) is an excellent biomarker of Bacillus anthracis (B. anthracis). The sensitive detection of DPA, especially through visual point-of-care testing, was significant for accurate and rapid diagnosis of anthrax to timely prevent anthrax disease or biological terrorist attack. Herein, a ratiometric fluorescent (R-FL) and electrochemiluminescent (ECL) dual-mode detection platform with a lanthanide ion-based metal-organic framework (Ln-MOF, i.e., M/Y-X: M = Eu, Y = Tb, and X = 4,4',4″-s-triazine-1,3,5-triyltri-m-aminobenzoic acid) was developed. Eu/Tb-TATAB nanoparticles were constructed to identify DPA. The R-FL detection platform quantitatively detected DPA by monitoring the I545/I617 ratio of the characteristic fluorescence peak intensities of Tb3+ ions and Eu3+ ions. The ECL sensing platform successfully quantified DPA by exploiting the burst effect of DPA on the ECL signal. The above methods had highly sensitive and rapid detection of DPA in water and serum samples. The results showed that this dual-mode detection platform may be projected to be a powerful instrument for preventing related biological warfare and bio-terrorism.

An overview on recent advances of reversible fluorescent probes and their biological applications

Due to the simplicity and low detection limit, fluorescent probes are widely used in both analytical sensing and optical imaging. Compared to conventional fluorescent probes, reversibility endows the reversible fluorescent probe outstanding advantages and special properties, making reversible fluorescent probes with capable of quantitative, repetitive or circulatory. Reversible fluorescent probes can also monitor the concentration dynamics of target analytes in real time, such as metal ions, proteins and enzymes, as well as intracellular redox processes, which have been widely applied in various fields. This review summarized the types and excellent properties of reversible fluorescent probes designed and developed in recent years. It also summarized the applications of reversible fluorescent probe in fluorescence imaging, biological testing, monitoring redox cycles, and proposed the remaining challenges and future development directions of the reversible fluorescent probe. This review provided comprehensive overview of reversible fluorescent probe, which may provide valuable references for the design and fabrication of the reversible fluorescent probe.

Paper-based ratiometric fluorescent sensing platform based on mixed quantum dots for the detection of glucose in urine

A paper-based ratiometric fluorescent sensing platform has been developed for glucose detection based on a dual-emission fluorescent probe consisting of carbon quantum dots (C QDs) and CdTe QDs. When the two kinds of QDs are mixed, the fluorescence of C QDs is reversibly quenched by CdTe QDs. However, in the presence of glucose, the fluorescence of CdTe QDs is quenched by H2O2 catalyzed by glucose oxidase (GOx), which restores the fluorescence of C QDs. The proposed paper-based ratiometric fluorescent sensing platform exhibited good sensitivity and selectivity towards glucose. The working linear range was 0.1 mM to 50 mM with a limit of detection (LOD) of 0.026 mM. Additionally, the proposed paper-based sensor possesses viability for the determination of glucose in actual urine samples.

A promising tool for clinical diagnostics: Dual-emissive carbonized polymer dots based cross-linking enhanced emission for sensitive detection of alkaline phosphatase and butyrylcholinesterase

Compared with single signal readout, dual-signal readout commendably corrects the impact of systematic or background error, achieving more accurate results for the diagnosis of many diseases. This work aimed to design and prepare dual-emissive fluorescent probes for the construction of ratiometric fluorescence biosensors to detect liver disease biomarkers. Sodium alginate (SA) with numerous potential sub-fluorophores and active sites and 4,4',4'',4'''-(porphine-5,10,15,20-tetrayl) tetrakis (benzoic acid) (TCPP) with macrocyclic conjugated structures were introduced to prepare the carbonized polymer dots (CPDs) with red/blue dual emission based on the cross-linking enhanced emission (CEE) effect and the luminescence of macrocyclic conjugated structures. The ratiometric fluorescence sensing systems were constructed by integrating the specific response of CPDs to Cu2+ and the affinity difference of Cu2+ to substrates or products of enzymes. The sensing systems, CPDs/Cu2+/PPi and CPDs/Cu2+/BTCh, were designed to detect liver disease biomarkers, alkaline phosphatase (ALP) and butyrylcholinesterase (BChE), respectively. The limit of detection for ALP and BChE was 0.35 U/L and 0.19 U/L, respectively. The proposed sensors were successfully applied to human serum samples from different health stages with satisfactory recoveries. These results demonstrate the successful design of a novel dual-emissive fluorescent probe and provide a feasible strategy for clinical detection.

Progress in optical sensors-based uric acid detection

The escalating number of patients affected by various diseases, such as gout, attributed to abnormal uric acid (UA) concentrations in body fluids, has underscored the need for rapid, efficient, highly sensitive, and stable UA detection methods and sensors. Optical sensors have garnered significant attention due to their simplicity, cost-effectiveness, and resistance to electromagnetic interference. Notably, research efforts have been directed towards UA on-site detection, enabling daily monitoring at home and facilitating rapid disease screening in the community. This review aims to systematically categorize and provide detailed descriptions of the notable achievements and emerging technologies in UA optical sensors over the past five years. The review highlights the advantages of each sensor while also identifying their limitations in on-site applications. Furthermore, recent progress in instrumentation and the application of UA on-site detection in body fluids is discussed, along with the existing challenges and prospects for future development. The review serves as an informative resource, offering technical insights and promising directions for future research in the design and application of on-site optical sensors for UA detection.

Ratiometric optical probes for biosensing

Biosensing by optical probes is bringing about a revolution in our understanding of physiological and pathological states. Conventional optical probes for biosensing are prone to inaccurate detection results due to various analyte-independent factors that can lead to fluctuations in the absolute signal intensity. Ratiometric optical probes provide built-in self-calibration signal correction for more sensitive and reliable detection. Probes specifically developed for ratiometric optical detection have been shown to significantly improve the sensitivity and accuracy of biosensing. In this review, we focus on the advancements and sensing mechanism of ratiometric optical probes including photoacoustic (PA) probes, fluorescence (FL) probes, bioluminescence (BL) probes, chemiluminescence (CL) probes and afterglow probes. The versatile design strategies of these ratiometric optical probes are discussed along with a broad range of applications for biosensing such as sensing of pH, enzymes, reactive oxygen species (ROS), reactive nitrogen species (RNS), glutathione (GSH), metal ions, gas molecules and hypoxia factors, as well as the fluorescence resonance energy transfer (FRET)-based ratiometric probes for immunoassay biosensing. Finally, challenges and perspectives are discussed.

Towards the Use of Individual Fluorescent Nanoparticles as Ratiometric Sensors: Spectral Robustness of Ultrabright Nanoporous Silica Nanoparticles

Here we address an important roadblock that prevents the use of bright fluorescent nanoparticles as individual ratiometric sensors: the possible variation of fluorescence spectra between individual nanoparticles. Ratiometric measurements using florescent dyes have shown their utility in measuring the spatial distribution of temperature, acidity, and concentration of various ions. However, the dyes have a serious limitation in their use as sensors; namely, their fluorescent spectra can change due to interactions with the surrounding dye. Encapsulation of the d, e in a porous material can solve this issue. Recently, we demonstrated the use of ultrabright nanoporous silica nanoparticles (UNSNP) to measure temperature and acidity. The particles have at least two kinds of encapsulated dyes. Ultrahigh brightness of the particles allows measuring of the signal of interest at the single particle level. However, it raises the problem of spectral variation between particles, which is impossible to control at the nanoscale. Here, we study spectral variations between the UNSNP which have two different encapsulated dyes: rhodamine R6G and RB. The dyes can be used to measure temperature. We synthesized these particles using three different ratios of the dyes. We measured the spectra of individual nanoparticles and compared them with simulations. We observed a rather small variation of fluorescence spectra between individual UNSNP, and the spectra were in very good agreement with the results of our simulations. Thus, one can conclude that individual UNSNP can be used as effective ratiometric sensors.

A ratiometric luminescence sensing platform based on lanthanide-based silica nanoparticles for selective and sensitive detection of Fe3+ and Cu2+ ions

Detection of Fe(III) and Cu(II) in water is highly desirable because their abnormal levels can cause serious harm to human health and environmental safety. In this work, a ratiometric luminescence sensing platform based on lanthanide-based silica nanoparticles was constructed for the detection of Fe³⁺ and Cu²⁺ ions. The terbium-silica nanoparticles (named SiO₂@Tb) with dual-emission signals were successfully prepared by grafting Tb³⁺ ions onto trimellitic anhydride (TMA) functionalized silica nanospheres. It can serve as a ratiometric fluorescent probe for the detection of Fe³⁺ and Cu²⁺ ions in water with the green emission of Tb³⁺ ions as a response signal and the blue emission of silica nanospheres as the reference signal. Significantly, an easy-to-differentiate color change for visual detection was also realized. SiO₂@Tb shows high sensitivity even in very low concentration regions towards the sensing of Fe³⁺ and Cu²⁺ with low detection limits of 0.75 μM and 0.91 μM, respectively. Moreover, the mechanism for the luminescence quenching of SiO₂@Tb was systematically investigated, and was attributed to the synergetic effect of the absorption competition quenching (ACQ) mechanism and cation exchange. This study demonstrates that SiO₂@Tb can be employed as a promising fluorescent probe for the detection of Fe³⁺ and Cu²⁺ ions, and the combination of lanthanide ions with silica nanoparticles is an effective strategy to construct a ratiometric fluorescent sensing platform for the determination of analytes in environmental detection.

Recent Advances in Silicon Quantum Dot-Based Fluorescent Biosensors

With the development of nanotechnology, fluorescent silicon nanomaterials have been synthesized and applied in various areas. Among them, silicon quantum dots (SiQDs) are a new class of zero-dimensional nanomaterials with outstanding optical properties, benign biocompatibility, and ultra-small size. In recent years, SiQDs have been gradually utilized for constructing high-performance fluorescent sensors for chemical or biological analytes. Herein, we focus on reviewing recent advances in SiQD-based fluorescent biosensors from a broad perspective and discussing possible future trends. First, the representative progress for synthesizing water-soluble SiQDs in the past decade is systematically summarized. Then, the latest achievement of the design and fabrication of SiQD-based fluorescent biosensors is introduced, with a particular focus on analyte-induced photoluminescence (fluorescence) changes, hybrids of SiQDs with other materials or molecules, and biological ligand-modification methods. Finally, the current challenges and prospects of this field are highlighted.

Intravital Microscopy Reveals Endothelial Transcytosis Contributing to Significant Tumor Accumulation of Albumin Nanoparticles

The principle of enhanced permeability and retention (EPR) effect has been used to design anti-cancer nanomedicines over decades. However, it is being challenged due to the poor clinical outcome of nanoparticles and controversial physiological foundation. Herein, we use a near-infrared-II (1000-1700 nm, NIR-II) fluorescence probe BPBBT to investigate the pathway for the entry of human serum albumin-bound nanoparticles (BPBBT-HSA NPs) into tumor compared with BPBBT micelles with phospholipid-poly (ethylene glycol) of the similar particle size about 110 nm. The plasma elimination half-life of BPBBT micelles was 2.8-fold of that of BPBBT-HSA NPs. However, the area under the BPBBT concentration in tumor-time curve to 48 h post-injection (AUC_tumor0→48h) of BPBBT-HSA NPs was 7.2-fold of that of BPBBT micelles. The intravital NIR-II fluorescence microscopy revealed that BPBBT-HSA NPs but not BPBBT micelles were transported from the tumor vasculature into tumor parenchyma with high efficiency, and endocytosed by the tumor cells within 3 h post-injection in vivo. This effect was blocked by cross-linking BPBBT-HSA NPs to denature HSA, resulting in the AUC_tumor0→48h decreased to 22% of that of BPBBT-HSA NPs. Our results demonstrated that the active process of endothelial transcytosis is the dominant pathway for albumin-bound nanoparticles' entry into tumor.

Ratiometric Temperature Sensing Using Highly Coupled Seven-Core Fibers

In this paper, a ratiometric approach to sensing temperature variations is shown using specialty fiber optic devices. We analyzed the transmission response of cascaded segments of multicore fibers (MCFs), and dissimilar lengths were found to generate an adequate scheme for ratiometric operation. The perturbation of optical parameters in the MCFs translates to a rich spectral behavior in which some peaks increase their intensity while others decrease their intensity. Thus, by selecting opposite-behavior peaks, highly sensitive ratiometric measurements that provide robustness against spurious fluctuations can be performed. We implemented this approach using seven-core fiber (SCF) segments of 5.8 cm and 9.9 cm. To test the system's response under controlled perturbations, we heated one of the segments from ambient temperature up to 150 °C. We observed defined peaks with opposite behavior as a function of temperature. Two pairs of peaks within the interrogation window were selected to perform ratiometric calculations. Ratiometric measurements exhibited sensitivities 6-14 times higher than single-wavelength measurements. A similar trend with enhanced sensitivity in both peak pairs was obtained. In contrast to conventional interferometric schemes, the proposed approach does not require expensive facilities or micrometric-resolution equipment. Moreover, our approach has the potential to be realized using commercial splicers, detectors, and filters.

Molecularly Imprinted Ratiometric Fluorescence Nanosensors

Molecularly imprinted ratiometric fluorescence (MIR-FL) nanosensors feature recognition selectivity, detection sensitivity, application universality, visualization accuracy, and device portability, and have gained popularity. However, the fluorescence intensity, nanostructure, color range, and practical application of the sensor still face severe difficulties to be solved. New strategies combined with various technologies have been developed to construct MIR-FL nanosensors for expanded applications. This Perspective highlights current resarch challenges and future prospects involving constructions and applications of MIR-FL nanosensors including dual-emission and triple-emission modes. The postimprinting mixing/modification strategies, microdevices, and multitarget detection are focused, and technology synergy, sensitivity/reproducibility improvement, application diversity/portability, etc. are proposed.

Dual-Color Fluorescent Hydrogel Microspheres Combined with Smartphones for Visual Detection of Lactate

Since it is difficult for human eyes to distinguish between two identical colors with only <15% variation in brightness, mono-color fluorescent hydrogel microspheres have some limitations in the detection of lactate. Herein, we prepared novel dual-color fluorescent hydrogel microspheres, which can achieve hue transformation. Microspheres were prepared by introducing a fluorescent nanoparticle as the reference signal while CdTe QDs were used as the response signal. We used smartphones with image processing software to collect and analyze data. In this way, the signal of lactate was converted to RGB (red, green, and blue) values, which can be quantitatively read. Within 10 to 1500 μM, the R/G values of the microspheres had a linear relationship with the logarithm of the lactate concentration. Moreover, color cards for lactate detection were prepared, from which the color change and concentration of lactate could be easily read by the naked eye. It is worth mentioning that this method was successfully applied to screen patients with hyperlactatemia.

Molecular Imprinted ZnS Quantum Dots-Based Sensor for Selective Sulfanilamide Detection

Combining molecular imprinted polymers and water-soluble manganese-doped zinc sulfide quantum dots (Mn²⁺: ZnS QDs), a new molecule imprinted polymers-based fluorescence sensor was designed. The molecule imprinted quantum dots (MIP@QDs) were constructed by coating molecular imprinted polymers layer on the surface of ZnS: Mn²⁺ QDs using the surface molecular imprinting technology. The developed MIP@QDs-based sensor was used for rapid and selective fluorescence sensing of sulfanilamide in water samples. The binding experiments showed that the MIP@QDs has rapid fluorescent responses, which are highly selective of and sensitive to the detection of sulfanilamide. The respond time of the MIP@QDs was 5 min, and the imprinting factor was 14.8. Under optimal conditions, the developed MIP@QDs-based sensor shows a good linearity (R² = 0.9916) over a sulfanilamide concentration range from 2.90 × 10⁻⁸ to 2.90 × 10⁻⁶ mol L⁻¹, with a detection limit of 3.23 × 10⁻⁹ mol L⁻¹. Furthermore, the proposed MIP@QDs-based sensor was applied to the determination of sulfanilamide in real samples, with recoveries of 96.80%–104.33%, exhibiting good recyclability and stability. Experimental results showed that the prepared MIP@QDs has the potential to serve as a selective and sensitive sensor for the fluorescence sensing of sulfonamides in water samples.

Multicolor Polystyrene Nanosensors for the Monitoring of Acidic, Neutral, and Basic pH Values and Cellular Uptake Studies

A first tricolor fluorescent pH nanosensor is presented, which was rationally designed from biocompatible carboxylated polystyrene nanoparticles and two analyte-responsive molecular fluorophores. Its fabrication involved particle staining with a blue-red-emissive dyad, consisting of a rhodamine moiety responsive to acidic pH values and a pH-inert quinoline fluorophore, followed by the covalent attachment of a fluorescein dye to the particle surface that signals neutral and basic pH values with a green fluorescence. These sensor particles change their fluorescence from blue to red and green, depending on the pH and excitation wavelength, and enable ratiometric pH measurements in the pH range of 3.0-9.0. The localization of the different sensor dyes in the particle core and at the particle surface was confirmed with fluorescence microscopy utilizing analogously prepared polystyrene microparticles. To show the application potential of these polystyrene-based multicolor sensor particles, fluorescence microscopy studies with a human A549 cell line were performed, which revealed the cellular uptake of the pH nanosensor and the differently colored emissions in different cell organelles, that is, compartments of the endosomal-lysosomal pathway. Our results demonstrate the underexplored potential of biocompatible polystyrene particles for multicolor and multianalyte sensing and bioimaging utilizing hydrophobic and/or hydrophilic stimuli-responsive luminophores.