"Turn-on" Pb2+ sensing and rapid detection of biothiols in aqueous medium and real samples

A naphthyl appended ninhydrin based colorimetric chemosensor for Cu2+ ion: Detection of cysteine and ATP

A ninhydrin-based colorimetric chemosensor (LH) was synthesized using 3-hydroxy-2-naphthoic hydrazide and 11H-indeno[1,2-b]quinoxalin-11-one. It was characterized by spectroscopic and single crystal X-ray diffraction techniques. In a semi-aqueous (MeOH/HEPES) system, LH displayed a characteristic chromogenic change from colorless to yellow upon adding Cu2+ ion, with the appearance of a new peak at λmax = 460 nm. A 1:1 binding stoichiometry between LH and Cu2+ ion has been found, with LOD = 2.3 μM (145 ppb) and LOQ = 8 μM (504 ppb). Based on experimental results the formula of [Cu(L)Cl(H2O)2] (1) was assigned and this in-situ generated 1 was found to exhibit a discoloration of upon gradual addition of cysteine (LOD = 60 nM) as well as ATP (LOD = 130 nM) having 1:2 and 1:1 stoichiometry respectively. The LH was useful for recognition of Cu2+ ion in real water samples and on filter paper strips. A two-input-two-output logic gate circuitry was also constructed by employing 1 and cysteine. The DFT/TDDFT calculations performed on LH and 1 were consistent with experimental findings. The binding affinity of LH towards HSA and BSA were determined with HSA having greater affinity than BSA, which was also supported by theoretical calculations.

Development of a novel fluorescent protein-based probe for efficient detection of Pb2+ in serum inspired by the metalloregulatory protein PbrR691

BACKGROUND

The accurate and rapid detection of blood lead concentration is of paramount importance for assessing human lead exposure levels. Fluorescent protein-based probes, known for their high detection capabilities and low toxicity, are extensively used in analytical sciences. However, there is currently a shortage of such probes designed for ultrasensitive detection of Pb2+, and no reported probes exist for the quantitative detection of Pb2+ in blood samples. This study aims to fill this critical void by developing and evaluating a novel fluorescent protein-based probe that promises accurate and rapid lead quantification in blood.

RESULTS

A simple and small-molecule fluorescent protein-based probe was successfully constructed herein using a peptide PbrBD designed for Pb2+ recognition coupled to a single fluorescent protein, sfGFP. The probe retains a three-coordinate configuration to identify Pb2+ and has a high affinity for it with a Kd' of 1.48 ± 0.05 × 10-17 M. It effectively transfers the conformational changes of the peptide to the chromophore upon Pb2+ binding, leading to fast fluorescence quenching and a sensitive response to Pb2+. The probe offers a broad dynamic response range of approximately 37-fold and a linear detection range from 0.25 nM to 3500 nM. More importantly, the probe can resist interference of metal ions in living organisms, enabling quantitative analysis of Pb2+ in the picomolar to millimolar range in serum samples with a recovery percentage of 96.64%-108.74 %.

SIGNIFICANCE

This innovative probe, the first to employ a single fluorescent protein-based probe for ultrasensitive and precise analysis of Pb2+ in animal and human serum, heralds a significant advancement in environmental monitoring and public health surveillance. Furthermore, as a genetically encoded fluorescent probe, this probe also holds potential for the in vivo localization and concentration monitoring of Pb2+.

A near-infrared fluorescent ratiometric probe with large Stokes shift for multi-mode sensing of Pb2+ and bioimaging

Pb2+ is a heavy metal ion pollutant that poses a serious threat to human health and ecosystems. The conventional methods for detecting Pb2+ have several limitations. In this study, we introduce a novel fluorescent probe that enables the detection of Pb2+ in the near-infrared region, free from interference from other common ions. A unique characteristic of this probe is its ability to rapidly and accurately identify Pb2+ through ratiometric measurements accompanied by a large Stokes shift of 201 nm. The limit of detection achieved by probe was remarkably low, surpassing the standards set by the World Health Organization, and outperforming previously reported probes. To the best of our knowledge, this is the first organic small-molecule fluorescent probe with both near-infrared emission and ratiometric properties for the detection of Pb2+. We present a triple-mode sensing platform constructed using a probe that allows for the sensitive and selective recognition of Pb2+ in common food items. Furthermore, we successfully conducted high-quality fluorescence imaging of Pb2+ in various samples from common edible plants, HeLa cells, Caenorhabditis elegans, and mice. Importantly, the probe-Pb2+ complex exhibited tumour-targeting capabilities. Overall, this study presents a novel approach for the development of fluorescent probes for Pb2+ detection.

Pb2+ Ion Sensors Employing Gold Etching Process: Comparative Investigation on Au Nanorods and Au Nanotriangles

The leaching phenomenon of gold (Au) nanomaterials by Pb2+ ions in the presence of 2-mercaptoethanol (2-ME) and thiosulfate (S2O32- ion) has been systematically applied to a Pb2+ ion sensor. To further investigate the role of Pb2+ ions in sensors containing Au nanomaterials, we revisited the leaching conditions for Au nanorods and compared them with the results for Au nanotriangles. By monitoring the etching rate, it was revealed that Pb2+ ions were important for the acceleration of the etching rate mainly driven by 2-ME and S2O32- pairs, and nanomolar detection of Pb2+ ions were shown to be promoted through this catalytic effect. Using the etchant, the overall size of the Au nanorods decreased but showed an unusual red-shift in UV-Vis spectrum indicating increase of aspect ratio. Indeed, the length of Au nanorods decreased by 9.4% with the width decreasing by 17.4% over a 30-min reaction time. On the other hand, the Au nanotriangles with both flat sides surrounded mostly by dense Au{111} planes showed ordinary blue-shift in UV-Vis spectrum as the length of one side was reduced by 21.3%. By observing the changes in the two types of Au nanomaterials, we inferred that there was facet-dependent alloy formation with lead, and this difference resulted in Au nanotriangles showing good sensitivity, but lower detection limits compared to the Au nanorods.

A multifunctional fluorescent probe based on Schiff base with AIE and ESIPT characteristics for effective detections of Pb2+, Ag+ and Fe3

In this work, three Schiff-based fluorescent probes with aggregation-induced emission (AIE) and excited intramolecular proton transfer (ESIPT) characters were synthesized by grafting 2-aminobenzothiazole group onto 4-substituted salicylaldehydes. More important, a rare tri-responsive fluorescent probe (SN-Cl) was developed by purposeful variation of substituents in the molecule. It could selectively identify Pb2+, Ag+ and Fe3+ in different solvent systems or with the help of masking agent and show complete fluorescence enhancement without interference of other ions. Meanwhile, the other two probes (SN-ON and SN-N) could only recognize Pb2+ in DMSO/Tris-HCl buffer (3: 7, v/v, pH = 7.4). According to Job's plot, density functional theory (DFT) calculations and NMR analysis, coordination between SN-Cl and Pb2+/Ag+/Fe3+ was determined. The LOD values for three ions were as low as 0.059 μM, 0.012 μM and 8.92 μM, respectively. Ideally, SN-Cl showed satisfactory performance in real water samples detection and test paper experiments for three ions. Also, SN-Cl could be used as an excellent imaging agent for Fe3+ in HeLa cells. Therefore, SN-Cl has the ability to be a "single fluorescent probe for three targets".

Development of ratiometric fluorescent probes based on peptides for sensing Pb2+ in aquatic environments and human serum

The detection of Pb²⁺ ions in aquatic environments and biofluid samples is crucial for assessment of human health. Herein, we synthesized two fluorescent probes (1 and 2) consisting of the peptide receptor for Pb²⁺ and a benzothiazolyl-cyanovinylene fluorophore that exhibited excimer-like emission when it aggregated. The peptide-based probes sensitively detected Pb²⁺ in purely aqueous solution (1% DMF) through ratiometric fluorescent response with a decrease in monomer emission at 520 nm and an increase in excimer emission at 570 nm. Specially, probe 2 showed remarkable detection features such as high selectivity for Pb²⁺over 15 metal ions, high binding affinity (K_d = 5.83 × 10^-7 M) for Pb²⁺, significant emission intensity changes, low detection limit (3.8 nM) of Pb²⁺, high water solubility, and visible light excitation (450 nm). Probe 2 was successfully used to quantify nanomolar concentration (0 ∼ 800 nM) of Pb²⁺ in real water samples (ground water and tap water). Specially, 2 was successfully applied for the quantification of Pb²⁺ in human serum by combination of microwave-assisted human serum digestion and filtration of digested serum by anion exchange cartridge. We clearly investigated the binding mode of 2 with Pb²⁺ using ¹H NMR, IR spectroscopy, pH titration, confocal microscopy, and size analysis. The peptide-based fluorescent probe might have great application potential for sensing Pb²⁺ in aquatic environments and biofluid samples.

Ratiometric Fluorescence Sensing System for Lead Ions Based on Self-Assembly of Bioprobes Triggered by Specific Pb2+-Peptide Interactions

Lead is one of the most toxic substances. However, there are few ratiometric fluorescent probes for sensing Pb2+ in aqueous solution as well as living cells because specific ligands for Pb2+ ions have not been well characterized. Considering the interactions between Pb2+ and peptides, we developed ratiometric fluorescent probes for Pb2+ based on the peptide receptor in two steps. First, we synthesized fluorescent probes (1-3) based on the tetrapeptide receptor (ECEE-NH2) containing hard and soft ligands by conjugation with diverse fluorophores that showed excimer emission when they aggregated. After investigation of fluorescent responses to metal ions, benzothiazolyl-cyanovinylene was evaluated as an appropriate fluorophore for ratiometric detection of Pb2+. Next, we modified the peptide receptor to decrease the number of hard ligands and/or to replace Cys with disulfide bond and methylated Cys for improving selectivity and cell permeability. From this process, we developed two fluorescent probes (3 and 8) among the probes (1-8) that exhibited remarkable ratiometric sensing properties for Pb2+ including high water solubility (≤2% DMF), visible light excitation, high sensitivity, selectivity for Pb2+, low detection limits (<10 nM), and fast response (<6 min). The binding mode study revealed that specific Pb2+-peptide interactions of the probes caused nanosized aggregates in which the fluorophores of the probes came close each other, exhibiting excimer emission. In particular, 8 based on tetrapeptide bearing a disulfide bond and two carboxyl groups with a good permeability successfully quantified intracellular uptake of Pb2+ in live cells through ratiometric fluorescent signals. The ratiometric sensing system based on specific metal-peptide interactions and excimer emission process could provide a valuable tool to quantify Pb2+ in live cells and pure aqueous solutions.

Ratiometric fluorescent detection of lead ions in aquatic environment and living cells using a fluorescent peptide-based probe

Ratiometric fluorescent detection using dual emission bands is highly necessary to quantify Pb(II) in aquatic environment and live cells. We synthesized a ratiometric fluorescent peptidyl probe (1) by conjugation of a peptide receptor for Pb(II) with an excimer-forming benzothiazolylcyanovinylene fluorophore. The peptidyl probe dissolved well in aqueous solution and displayed an emission band at 538 nm (λ_ex = 460 nm). Upon addition of Pb(II) (0-20 μM), the emission maximum shifted from 538 nm to 575 nm and the emission intensity ratio (I₅₇₅ /I₅₃₈) increased significantly from 0.40 to 2.26. 1 exhibited a selective ratiometric response to Pb(II) over other metal ions. 1 with a low detection limit (1.2 ppb) of Pb(II) detected nanomolar concentrations (0-500 nM) of Pb(II) ions in groundwater and tap water. The cell-permeable probe detected intracellular Pb(II) by ratiometric fluorescent images. The binding mode study using NMR, IR and CD spectroscopy, and TEM revealed that the probe formed a 1:1 complex with Pb(II) and then formed red-emissive nanoparticles and fibrils. The probe exhibited desirable detection properties such as ratiometric detection, high solubility in water, visible light excitation, high selectivity and sensitivity for Pb(II), cell-permeability, and rapid response (< 6 min).

A pyridine based Schiff base as a selective and sensitive fluorescent probe for cadmium ions with “turn-on” fluorescence responses

A pyridine based Schiff base probe (PMPA) showed high selectivity and sensitivity towards Cd2+ ions with intense bluish green fluorescence. The sensing mechanism of probe PMPA for detecting Cd2+ was based on the inhibited PET and CHEF processes.

A simple and rapid fluorescent approach for Pb2+ determination and application in water samples and living cells

A novel selective fluorescent chemosensor, thiosemicarbazide-appended naphthalimide derivative (TND), has been designed and synthesized, which exhibited good selectivity and sensibility for Pb²⁺ in CH₃CN:H₂O (1:1) solution. The probe TND showed obvious color changes under UV light of 365 nm and displayed turn-on fluorescence response with Pb²⁺ added. The binding mode of TND with Pb²⁺ was found to be 1:1 based on the Job's plot analysis. The detection limit of Pb²⁺ was 4.7 nM, which is far below the allowable concentration determined by WHO in drinking water. Moreover, the fortified recoveries of Pb²⁺ were from 100.54% to 113.68% in water samples. TND is also applied for fluorescence imaging of Pb²⁺ in lysosomes of human stromal cell line (HSC). This study indicated that TND would be a potential sensor detecting Pb²⁺ in real sample.

An Acylhydrazone-Based Fluorescent Sensor for Sequential Recognition of Al3+ and H2PO4. MATERIALS (BASEL, SWITZERLAND) 2021;14:6392. [PMID: 34771920 PMCID: PMC8585233 DOI: 10.3390/ma14216392]

A novel acylhydrazone-based fluorescent sensor NATB was designed and synthesized for consecutive sensing of Al3+ and H2PO4-. NATB displayed fluorometric sensing to Al3+ and could sequentially detect H2PO4- by fluorescence quenching. The limits of detection for Al3+ and H2PO4- were determined to be 0.83 and 1.7 μM, respectively. The binding ratios of NATB to Al3+ and NATB-Al3+ to H2PO4- were found to be 1:1. The sequential recognition of Al3+ and H2PO4- by NATB could be repeated consecutively. In addition, the practicality of NATB was confirmed with the application of test strips. The sensing mechanisms of Al3+ and H2PO4- by NATB were investigated through fluorescence and UV-Visible spectroscopy, Job plot, ESI-MS, 1H NMR titration, and DFT calculations.

Platform- and label-free detection of lead ions in environmental and laboratory samples using G-quadraplex probes by circular dichroism spectroscopy

Guanine-rich quadruplex (G-QD) are formed by conversion of nucleotides with specific sequences by stabilization of positively charged K⁺ or Na⁺. These G-QD structures differentially absorb two-directional (right- and left-handed) circularly polarized light, which can discriminate the parallel or anti-parallel structures of G-QDs. In this study, G-QDs stabilized by Pb²⁺ were analyzed by a circular dichroism (CD) spectroscopy to determine Pb²⁺ concentration in water samples. Thrombin aptamer (TBA), PS2.M, human telomeric DNA (HTG), AGRO 100, and telomeric related sequence (T2) were studied to verify their applicability as probes for platform- and label-free detection of Pb²⁺ in environmental as well as laboratory samples. Among these nucleotides, TBA and PS2.M exhibited higher binding constants for Pb²⁺, 1.20-2.04 × 10⁶/M at and 4.58 × 10⁴-1.09 × 10⁵/M at 100 micromolar and 100 mM K⁺ concentration, respectively. They also exhibited excellent selectivity for Pb²⁺ than for Al³⁺, Cu²⁺, Ni²⁺, Fe³⁺, Co²⁺, and Cr²⁺. When Pb²⁺ was spiked into an effluent sample from a wastewater treatment plant (WWTP), its existence was detected by CD spectroscopy following a simple addition of TBA or PS2.M. By the addition of TBA and PS2.M, the Pb²⁺ signals were observed in effluent samples over 0.5 micromolar (100 ppb) concentration. Furthermore, PS2.M caused a Pb²⁺-specific absorption band in the effluent sample without spiking of Pb²⁺, and could be induced to G-QD structure by the background Pb²⁺ concentration in the effluent, 0.159 micromolar concentration (3.30 ppb). Taken together, we propose that TBA and PS2.M are applicable as platform- and label-free detection probes for monitoring Pb²⁺ in environmental samples such as discharged effluent from local WWTPs, using CD spectroscopy.

Ratiometric fluorescent nanoprobes based on Resonance Rayleigh Scattering and inner filter effect for detecting alizarin red and Pb2

A new ratiometric fluorescent strategy for detection of alizarin red (ARS) was designed based on the fluorescence of CDs and scattered light of scatterer. The CDs-ARS system can be used to detect Pb²⁺ based on that the complexation between ARS and Pb²⁺. With the addition of ARS, the fluorescence of CDs was apparently quenched via inner filter effect (IFE). Resonance Rayleigh Scattering (RRS) at 350 nm was enhanced by an increase in the number of scatterer. The value of ln(I₃₅₀/I₄₂₅) was linearly correlated with ARS concentration in the range of 0-80 μM, and the detection limit for ARS was calculated to be 68.1 nM. When Pb²⁺ was added to the CDs-ARS system, the complexation of ARS with Pb²⁺ increased the size of the scatterer, resulting in the increase of the RRS intensity at 350 nm. Due to the affinity between ARS and Pb²⁺, the overlap of the emission spectra of CDs and the absorption spectra of ARS was reduced, resulting in the IFE effect was inhibited and the recovery of the fluorescence of CDs. The value of I₃₅₀/I₄₂₅ linearly increased with the addition of Pb²⁺ within the range of 10-50 μM, the limit of detection was 36.8 nM. As for practical application, CDs and CDs-ARS were applied to detect ARS and Pb²⁺ in tap water and poor water, respectively. The recovery values were obtained to be 95.4-98.8% and 93.4-101.7%. Furthermore, the system of CDs-ARS has been successfully applied to H1299 cell imaging.

Nanomolar detection of biothiols via turn-ON fluorescent indicator displacement

A simple, visual colour and turn-ON fluorescent method for the detection of biothiols under physiological conditions is reported. The chemosensing is achieved on the basis of the displacement of BODIPY dyes from the surface of gold nanoparticles.