A novel hydrazide Schiff base self-assembled nanoprobe for selective detection of human serum albumin and its applications in renal disease surveillance

Design and development of a bright NIR fluorescent probe for selective HSA detection in human blood serum and urine

Human serum albumin (HSA), an important human blood protein, plays a critical role in maintaining osmotic pressure and facilitating the transport of various substances. Abnormal HSA levels are associated with diseases like kidney disease, heart problems, diabetes, and liver damage, necessitating the development of accurate methods for HSA detection. This paper describes the design, synthesis, and evaluation of four BODIPY-based near-infrared (NIR) fluorescent probes (BD1-BD4) for the selective detection of HSA. Among the synthesized probes, BD1 demonstrated exceptional sensitivity and specificity, exhibiting a 147-fold fluorescence enhancement at 660 nm (λex = 600 nm) with a Stokes shift of 60 nm. The probe achieved a low detection limit of 9.5 nM, enabling the effective quantification of HSA in complex biological samples such as human blood serum and artificial urine. Competitive binding studies using ibuprofen confirmed that BD1 binds selectively to binding site II of HSA, which was further supported by a molecular docking study. Additionally, BD1 demonstrated HSA detection with a high recovery rate in artificial urine.

Selective Sensing of Human Serum Albumin by a D···π···A-Based Self-Assembled Nanoprobe via a Disassembly Mechanism: Quantification and Live Cell Imaging Applications

Human serum albumin (HSA) serves as a vital biomarker for diagnosing kidney and liver diseases. Herein, we describe the detailed synthetic procedure of a D···π···A-based fluorescent probe HPQM [(E)-2-(2-(8-(3-hydroxypropoxy)-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)vinyl)-1,1,3-trimethyl-1H-benzo[e]indol-3-ium], which forms weakly fluorescent nanoaggregates in aqueous environments due to the aggregation-caused quenching phenomenon. Nevertheless, in the presence of HSA, it becomes highly fluorescent (∼25-fold enhancement), facilitating the disassembly of nanoaggregates into its monomer. The study reveals that noncovalent interactions between HPQM and HSA promote the disassembly of the nanoaggregates and trapping of the HPQM monomer in the HSA cavity, resulting in red fluorescence by prohibiting the free rotation of the probe molecules owing to the substantial steric hindrance and low polarity environment present inside the HSA cavity. Multiple circular dichroism spectral analyses of HSA in the absence and presence of the probe revealed HPQM-induced conformational changes in the secondary and tertiary structures of HSA. The 3σ/slope technique yielded a limit of detection value of 29.8 nM (1.98 mg/L) in aqueous solutions, highlighting its high sensitivity toward HSA. Also, HPQM offers some practical utility including live cell imaging of HSA and the detection and estimation of HSA content in human blood and urine.

A red-emissive tripodal nanoprobe for the discrimination of serum albumin with conformational change from Y- to ω-like and probing mitochondrial viscosity

Serum albumins, such as bovine serum albumin (BSA) and human serum albumin (HSA), play a crucial role in various biological processes. Discrimination between BSA and HSA is challenging due to their similar structures and reactivity. Here, we report mitochondria-targeted red fluorescent tripodal nanoprobe DMAS-TP, forming spherical nano-aggregates (∼90 nm) in water for discrimination of BSA over HSA and detection and imaging of viscosity in HeLa cells. DMAS-TP exhibits an ∼56-fold fluorescence intensity increase in 95% glycerol compared to water, which indicates restricted movement in high-viscosity solvents. Furthermore, the addition of 5 equiv. BSA and HSA to DMAS-TP solution displays ∼50-fold and ∼10-fold fluorescence intensity increases at 630 nm (λex 490 nm), respectively, and can detect as low as 20 nM BSA and 140 nM HSA. The fluorescence anisotropy plot shows that the DMAS-TP anisotropy values decrease from 0.234 to 0.185 with increasing [BSA/HSA], suggesting a conformational change from Y- to ω-like structure. The fluorescence lifetime of DMAS-TP increases gradually with BSA/HSA, suggesting dynamic complexation. DMAS-TP aggregates diminish to 6 nm particles when encapsulated in BSA confirmed by DLS, SEM, and TEM, with reduced fluorescence intensity in the presence of bilirubin, indicating that DMAS-TP binds within the BSA cavity near site IB. DMAS-TP is highly permeable to HeLa cells and shows a strong affinity for mitochondria, making it suitable for imaging viscosity and BSA via strong fluorescence in the red channel.

A-D-A type fluorescent probe with dual quaternary-ammonium-salt anchors for turn on detection of HSA in wide emission gamut

Human serum albumin (HSA) is a key protein implicates in various physiological and pathological conditions such as renal injury, diabetes mellitus. Herein, we report an AIE-active fluorescent probe (DNI-4) for detection of HSA with a "turn on" response covering visible and near-infrared region (500 - 800 nm). Combining with a triphenylamine and two 1,8-naphthalimide moieties, the chromophore segment of DNI-4 forms a "A-D-A" type molecular architecture with the twisted intramolecular charge transfer property. Two quaternary ammonium salt moieties are introduced into the chromophore to give the probe (DNI-4), which has good hydrophilicity and can interact with HSA to form the dye-HSA aggregates with "turn-on" signal. DNI-4 demonstrates a good linear correlation over a low concentration range of HSA from 0 to 0.2 μM (R2 = 0.9995), with a limit of detection (LOD) as low as 15 nM. We tested the diameters and potential values of DNI-4 and HSA to disclose the variation in microstructure before and after the recognition event. Furthermore, we test and compare the sensitivity and association constants of DNI-4 and two control compounds, neutral DNI-1 and mono-quaternary-ammonium-salt-substituted DNI-5. The results indicate the electronic interaction is a key factor for recognition and DNI-4 with the most positive groups is the best probe for HSA. At last, DNI-4 is successfully applied to probe HSA in the Hela cells indicating the potential application in fluorescent sensing and bioimaging.

A self-assembled fluorescent nanoprobe recognized by FA1 site for specifically selecting HSA: Its applications in hemin detection, cell imaging and fluorescent tracing drug delivery

As naturally essential biomacromolecule, HSA has become diagnostic indicators for various diseases and universal carriers for anticancer drug delivery, therefore, fluorescence detection and labeling for HSA possess significant application value in the biomedical field. In this paper, hydrazide Schiff base fluorescent probe NDQC was designed and synthesized, which self-assembled into nanoparticles in aqueous solution system and demonstrated excellent selectivity and sensitivity towards HSA. Through displacement assay and molecular docking simulation, the binding of NDQC with HSA in FA1 site was demonstrated, thereby no obvious fluorescence signal presented for homologous protein BSA due to their structural differences in binding site. Non-toxic probe NDQC is suitable for the fluorescence imaging of HSA in cells, and colocalization fluorescence images showed that NDQC-HSA could illuminate mitochondria. Based on the pH sensitivity of fluorescence emission for NDQC-HSA, discrimination of cancer cells and normal cells could be achieved. For practical applications, NDQC-HSA can be employed to measure the content of hemin. More importantly, NDQC could fluorescently label HSA and therefore NDQC-HSA complex act as the carrier for loading cisplatin. The present findings demonstrate that the probe NDQC has potential in exploring HSA at cellular levels and hold great promise in application of tracking drug-loading nanoparticles.

New molecular mechanism of nanoplastics affecting cadmium protein toxicity: Conformational response and differential binding of human serum albumin

The significant health risks of nanoplastics (NPs) and cadmium (Cd) are currently attracting a great deal of attention and research. At present, the effects and mechanisms of NPs and Cd on human serum albumin (HSA), a key functional protein in the organism on transportation, remain unknown. Here, the differences in the effects and mechanisms of action of Cd alone and composite systems (NPsCd) were explored by enzyme activity assay, multi-spectroscopy analysis and molecular docking. The results showed that HSA activity was inhibited and decreased to 80 % and 69.55 % (Cd = 30 mg/L) by Cd alone and NPs-Cd exposure, respectively. Exposure to Cd induced backbone disruption and protein defolding of HSA, and secondary structure disruption was manifested by the reduction of α-helix. Cd exposure also induces fluorescence sensitization of HSA. Notably, the addition of NPs further exacerbated the effects associated with Cd exposure, which was consistent with the changes in HSA activity. Thus, the above conformational changes may be responsible for inducing the loss of enzyme activity. Moreover, it was determined by RLS spectroscopy that NPs-Cd bound to HSA in the form of protein crowns. Molecular docking has further shown that Cd binds to the surface of Sudlow site II of HSA, suggesting that Cd impairs the function of HSA by affecting the protein structure. More importantly, the addition of NPs further exacerbated the disruption of the protein structure by the adherent binding of HSA on the surface of the plastic particles, which induced a greater change in the enzyme activity. This study provides useful perspectives for investigating the impact of composite pollution on HSA of human functional proteins.

Sensitive and accurate monitoring of urinary albumin and point-of-care testing using a fluorescent probe with anti-interference capacity against exogenous drugs

Fluorescent probes have been reported for monitoring urinary albumin (u-ALB) to enable early diagnosis of kidney diseases and facilitate regular point-of-care testing (POCT) for chronic kidney disease (CKD) patients. However, the albumin can bind hydrophobic drugs through host-guest interactions, which may result in decreased accuracy of probes at regular drug sites and hamper POCT of albuminuria since CKD patients often need to take medications routinely. Herein, we reported a novel fluorescent probe (NC-2) by molecular engineering of a reported AIEgen (NC-1). The introduction of a non-conjugated ring moiety to the molecular rotor granted the NC-2 enhanced sensitivity with a limit of detection in urine of 8.7 mg/L, which is below l the threshold of microalbuminuria (30 mg/L). Moreover, the NC-2 was found to preferentially bind to the FA1 site of ALB, conferring it with excellent anti-interference capacities against exogenous drug molecules and metabolites. Simulation experiments using lab-spiked urine samples containing common drugs taken by CKD patients demonstrated that the probe could provide satisfied detecting accuracy (80-90 %). Furthermore, a paper-based device was constructed and achieved on-site detection of u-ALB in qualitative and semi-quantitative manners. Findings in this work were of great significance to the development of fluorescent probes for accurate detection of ALB in complex urine samples and the further achievement of fluorescence-based POCT for CKD.

A fluorescent "Turn-ON" probe with rapid and differential response to HSA and BSA: quantitative detection of HSA in urine

The present study provides insight into the differential response of a benzimidazole-malononitrile fluorescent "Turn-ON" probe on interaction with two structurally similar proteins, BSA and HSA. Compound 6 shows more sensitivity towards the two SAs, which is completely lost in the case of compound 7, synthesized by substitution on 6. The aggregates of compound 6 show absorption maxima at 385 nm and weak emission maxima at 565 nm. Compound 6 forms a new emission band at 475 nm on gradual addition of BSA (200 μM) along with a slight increase in the emission band at 565 nm. However, on addition of HSA (50 μM), a new band at 475 nm is formed. In contrast to BSA, in the case of HSA, 50% quenching is observed in the emission band of compound 6 at 565 nm. The new band formed on the interaction of 6 with BSA shows four-fold more enhancement compared to HSA. Furthermore, the mechanism of interaction of 6 with serum albumin has been investigated through lifetime-fluorescence analysis, site-selective drug experiments, dynamic light scattering, FE-SEM, FT-IR, etc. Molecular docking studies and site marker drug displacement experiments reveal differential interactions of 6 towards the two structurally similar proteins. Aggregates of 6 with an average hydrodynamic size of 100-190 nm are disassembled on adding BSA and HSA, and the size of the serum albumin and 6 complex decreases to 10-20 nm, revealing the ligand's encapsulation in the serum albumin cavity. Practical applicability for the quantitative detection of HSA in human urine samples is also demonstrated. The high binding affinity, sensitivity, selectivity and differential response of probe 6 towards two serum albumins (HSA and BSA) and significant quantification of HSA in urine samples shows the potential ability of this probe in medical applications.

Design and synthesis of a TICT-based red-emissive fluorescent probe for the rapid and selective detection of HSA in human biofluids and live cell imaging

Here, we report the design and synthesis of a D⋯π⋯A-based fluorescent probe, (E)-4-(4-(dibutylamine)-2-hydroxystyryl)-1-methylquinolin-1-ium (DHMQ), which is nonfluorescent in ∼100% PBS buffer medium due to a twisted intra molecular charge transfer (TICT) phenomenon and it becomes highly fluorescent (∼149 fold) in the presence of human serum albumin (HSA), owing to the restriction of its intramolecular free rotation inside the hydrophobic binding cavity of HSA. The site-selective fluorescence displacement assay and molecular docking studies clearly reveal that DHMQ selectively binds at subdomain IB of HSA. The 3σ/slope method was adopted to determine the limit of detection (LOD) value, which was as low as 2.39 nM in ∼100% PBS medium, indicating its high sensitivity towards HSA. The low dissociation constant value [Kd = (1.066 ± 0.017) μM] suggests a strong complexation between the DHMQ and HSA. Importantly, it has been demonstrated that DHMQ is capable of detecting HSA in real human serum and urine samples and was found to be suitable for live cell imaging of HSA.

1,4-Dihydropyridine-based FA1 site-specific fluorescent probes for the selective detection and quantification of HSA levels in biofluids

Human serum albumin (HSA) is a multifunctional circulatory protein essential for many physiological processes including oncotic pressure maintenance, ligand/drug binding and transport, antioxidant activity, etc. Abnormal HSA levels in biological fluids have been reported in a variety of clinical disorders, making it a potential biomarker for early diagnosis. Low serum albumin levels have been linked to increased long- and short-term mortality rates in ICU patients. Therefore, quantifying HSA in biofluids such as serum and urine offers a convenient approach for the early identification of underlying clinical conditions and assessing the risk factors. Herein, we report a series of fluorescent 1,4-dihydropyridine (DHP) derivatives for the detection and quantification of HSA in biofluids. Their response towards HSA can be tuned by varying the substituents at the C-4 and the N-1 of the DHP ring. Depending on the nature of the substituents, they generated either a turn-on or ratiometric response with a LoD in low nanomolar or subnanomolar levels. A pair of enantiomers obtained by introducing a chiral center on the N-substituents highlighted the importance of stereochemistry in HSA-ligand interactions. Quantification of HSA in complex biofluids, such as blood serum and urine, was also accomplished using these probes. The high selectivity of some of the probes towards HSA over the homologous BSA allowed the discrimination of these two proteins. The preferred binding location of the probes was the hemin binding site and the detection mechanism was identified as the restriction of intramolecular rotation. Additionally, a prototype of a smartphone-integrated point-of-care device was also fabricated to demonstrate the feasibility of utilizing these probes in clinical settings.

Advancements in molecular disassembly of optical probes: a paradigm shift in sensing, bioimaging, and therapeutics

The majority of self-assembled fluorescent dyes suffer from aggregation-caused quenching (ACQ), which detrimentally affects their diagnostic and therapeutic effectiveness. While aggregation-induced emission (AIE) active dyes offer a promising solution to overcome this limitation, they may face significant challenges as the intracellular environment often prevents aggregation, leading to disassembly and posing challenges for AIE fluorogens. Recent progress in signal amplification through the disassembly of ACQ dyes has opened new avenues for creating ultrasensitive optical sensors and enhancing phototherapeutic outcomes. These advances are well-aligned with cutting-edge technologies such as single-molecule microscopy and targeted molecular therapies. This work explores the concept of disaggregation-induced emission (DIE), showcasing the revolutionary capabilities of DIE-based dyes from their design to their application in sensing, bioimaging, disease monitoring, and treatment in both cellular and animal models. Our objective is to provide an in-depth comparison of aggregation versus disaggregation mechanisms, aiming to stimulate further advancements in the design and utilization of ACQ fluorescent dyes through DIE technology. This initiative is poised to catalyze scientific progress across a broad spectrum of disciplines.

A microenvironment-sensitive red emissive probe with a large Stokes shift for specific recognition and quantification of serum albumin in complex biofluids and live cells

Human serum albumin (HSA) is regarded as a useful biomarker for rapid medical diagnosis of various disorders mainly related to the kidneys and liver. Hence, it is crucial to identify and monitor the HSA level in complex biofluids (urine and blood samples) using a simple approach. Herein, we have designed and synthesized an intramolecular charge transfer (ICT) based environment-sensitive fluorescent molecular probe, (E)-2-(3-(2-(5-methoxy-1H-indol-3-yl)vinyl)-5,5-dimethylcyclohex-2-en-1-ylidene)malononitrile (DCI-MIN), that can selectively interact with HSA in PBS buffer solution and exhibit a ∼78-fold enhancement in fluorescence intensity with a significant Stokes shift (∼126 nm), which is important to avoid interference from the excitation light. The significant red fluorescence response can be attributed to the suppression of free intramolecular rotation of the DCI-MIN probe inside the hydrophobic binding cavity of HSA and the low polar microenvironment present within HSA. According to the 3σ/slope method, the detection limit was found to be 1.01 nM (0.0671 mg L-1) in aqueous solutions, which is significantly lower than the normal level of HSA in healthy urine and blood serum, indicating its high sensitivity. DCI-MIN has the ability to exhibit useful applications, including the detection and quantification of HSA concentration in complex biofluids (human urine and blood samples) as well as the imaging of serum albumin in living cells.

NIR-I emissive cyanine derived molecular probe for selective monitoring of hepatic albumin levels during hyperglycemia

In this study, we report a small molecule optical marker BI-CyG derived from the structural engineering of a cyanine scaffold. The developed probe offers suitable advantages over existing cyanine-based albumin specific probes in terms of its excitation and emission wavelengths, which are 760 and 830-832 nm, respectively. Structural tuning of the cyanine architecture leading to extended π-conjugation and resulting in a suitable bathochromic shift in the emission wavelength of the probe is represented in this study. The probe besides emitting in the NIR region, also possesses the desirable characteristics of being a potential target selective optical marker, as established from various biophysical studies. Molecular modelling and simulation studies provided critical insights into the binding of the probe in the protein microenvironment, which was further supported by experimental studies. The probe displayed intracellular albumin selectivity and was utilized for demonstrating alteration in albumin levels in pathological states such as hyperglycemia in hepatic cells. The present study also sheds some light on using BI-CyG as an imaging probe and on the role of metformin as a suitable drug for balancing hyperglycemia-induced reduced intra-hepatic albumin levels. The study, thus, attempts to highlight the structural derivatization of cyanine to afford a potential probe for serum albumin and its deployment to image altering albumin levels in an induced pathological condition, hyperglycemia.

Development of Human Serum Albumin Fluorescent Probes in Detection, Imaging, and Disease Therapy

Human serum albumin (HSA) acts as a repository and transporter of substances in the blood. An abnormal concentration may indicate the occurrence of liver- and kidney-related diseases, which has attracted people to investigate the precise quantification of HSA in body fluids. Fluorescent probes can combine with HSA covalently or noncovalently to quantify HSA in urine and plasma. Moreover, probes combined with HSA can improve its photophysical properties; probe-HSA has been applied in real-time monitoring and photothermal and photodynamic therapy in vivo. This Review will introduce fluorescent probes for quantitative HSA according to the three reaction mechanisms of spatial structure, enzymatic reaction, and self-assembly and systematically introduce the application of probes combined with HSA in disease imaging and phototherapy. It will help develop multifunctional applications for HSA probes and provide assistance in the early diagnosis and treatment of diseases.

Development of a High-Accuracy, Low-Cost, and Portable Fluorometer with Smartphone Application for the Detection of Urinary Albumin towards the Early Screening of Chronic Kidney and Renal Diseases

This study presents the development of a portable fluorometer with a smartphone application designed to facilitate the early screening of chronic kidney and renal diseases by enabling the sensitive detection of urinary albumin. Utilizing a fluorescence-based aptasensor, the device achieved a linear calibration curve (0.001-1.5 mg/mL) with a linearity of up to 0.98022 and a detection limit of 0.203 µg/mL for human serum albumin (HSA). The analysis of 130 urine samples demonstrated comparable performance between this study's fluorometer, a commercial fluorometer, and the standard automated method. These findings validate the feasibility of the portable fluorometer and aptasensor combination as a reliable instrument for the sensitive and specific measurement of HSA in urine samples. Moreover, the fluorometer's portability offers potential applications in portable point-of-care testing, enhancing its utility in clinical settings for early disease screening.

Chiral 8-aminoBODIPY-based fluorescent probes with site selectivity for the quantitative detection of HSA in biological samples

Human serum albumin (HSA) is one of the vital proteins in blood serum, and its optimum level is a reflection of the physiological well-being of an individual. Any abnormalities in serum HSA levels could often be a sign of disguised physiological disorders. The importance of fast and accurate determination of serum HSA levels has led to the development of various quantification methods. Among these, fluorescence-based methods employ molecular probes capable of producing selective responses on interaction with HSA. Herein, we report chiral 8-aminoBODIPY-based probes having blue emission for the quantitative detection of HSA in buffer and human blood serum. A pair of 8-aminoBODIPY enantiomers, namely R-PEB and S-PEB, were synthesized. They exhibited a fast 'turn-on' fluorescence response towards HSA, allowing its detection and quantification. In PBS buffer, R-PEB and S-PEB showed very good sensitivity with a limit of detection (LoD) of 25 nM (K_D = 9.84 ± 0.14 μM) and 39 nM (K_D = 18.67 ± 0.21 μM), respectively. The linear relationship observed between the fluorescence intensity of R-PEB/S-PEB and the HSA concentration in serum samples allowed us to generate a reference curve for HSA estimation for practical applications. Examination of unknown serum samples showed a good correlation with the results obtained by the benchmark BCG method. Interestingly, the difference in these probes' dissociation constants and LoD indicated their differential binding to HSA. Considering the availability of multiple ligand binding sites in HSA, their binding preferences were investigated in detail by displacement assays using site-specific drugs. These studies showed the preferential affinity of R-PEB towards site II, which was further substantiated using molecular docking studies. However, these displacement assays could not identify the preferred binding site of S-PEB. Blind docking studies indicated that S-PEB occupied a site closer to FA5. Selective binding of R-PEB to site II and its characteristic photophysical response can be utilized to quickly screen potential site II binding drugs.

Serum Albumin Inspired Self-Assembly/Disassembly of a Fluorogenic Nanoprobe for Real-Time Monitoring and Quantification of Urinary Albumin with Live Cell Imaging Application

Abnormal levels (high/low) of urinary human serum albumin (HSA) are associated with a number of diseases and thus act as an essential biomarker for quick therapeutic monitoring and biomedical diagnosis, entailing the urgent development of an effective chemosensor to quantify the albumin levels. Herein, we have rationally designed and developed a small fluorogenic molecular probe, (Z)-2-(5-((8-hydroxy-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl) methylene)-4-oxo-2-thioxothiazolidin-3-yl) acetic acid (HJRA) with a twisted intramolecular charge transfer (TICT) property, which can easily self-assemble into nonfluorescent nanoaggregates in aqueous solution. However, HJRA nanoaggregates can selectively bind with serum albumin proteins (HSA/BSA) in ∼100% PBS medium, thereby facilitating the disassembly of nanoaggregates into monomers, exhibiting a clear turn-on red fluorescent response toward HSA and BSA. Analysis of the specific binding mechanism between HJRA and HSA using a site-selective fluorescence displacement assay and molecular docking simulations indicates that a variety of noncovalent interactions are responsible for the disassembly of nanoaggregates with the concomitant trapping of the HJRA monomer at site I in HSA, yielding a substantial red emission caused by the inhibition of intramolecular rotation of HJRA probe inside the hydrophobic cavity of HSA. The limit of detection (LOD) determined by the 3σ/slope method was found to be 1.13 nM, which is substantially below the normal HSA concentration level in healthy urine, signifying the very high sensitivity of the probe toward HSA. The comparable results and quick response toward quantification of HSA in urine by HJRA with respect to the Bradford method clearly point toward the superiority of this method compared to the existing ones and may lead to biomedical applications for HSA quantification in urine. It may also find potential application in live-cell imaging of HSA.

Dual-Channel Recognition of Human Serum Albumin and Glutathione by Fluorescent Probes with Site-Dependent Responsive Features

Design of chemical probes with high specificity and responses are particularly intriguing. In this work, a fluorescent probe (M-OH-SO₃) with dual-channel spectral responses toward human serum albumin (HSA) is presented. By employing dinitrobenzenesulfonate as a recognition site as well as a fluorescence quencher, probe M-OH-SO₃ displayed weak fluorescence, which, nevertheless, exhibits extensive yellow (575 nm) and red (660 nm) fluorescence emissions toward HSA under excitations at 400 and 500 nm, respectively. Interestingly, M-OH-SO₃ displayed the best performance toward HSA with distinctly higher selectivity than that of its counterparts M-SO₃, M-H-SO₃, and M-F-SO₃, which were prepared simply by modulating the functional group at the ortho position of the dicyanoisophorone core. Molecular docking results revealed that M-OH-SO₃ possesses the lowest binding energy among the tested derivatives and accordingly the strongest binding affinity. Probe M-OH-SO₃ showed a good linear relationship toward HSA in a range of 0.5-18 μM with a limit of detection of 35 nM. Cell imaging results demonstrated that probe M-OH-SO₃ could visualize the variation HSA levels in hepatocarcinoma cells. In addition, probe M-OH-SO₃ could also be employed for the recognition of glutathione through the cleavage of the dinitrobenzenesulfonate group along with an enhancement of emission at 575 nm. The site-dependent properties inspired a novel paradigm for design of fluorescent probes with optimized selectivity and responses.

Rationally designed far-red emitting styryl chromones and a magnetic nanoconjugate for strip-based 'on-site' detection of metabolic markers

The global burden of liver damage and renal failure necessitates technology-aided evolution towards point-of-care (POC) testing of metabolic markers. Hence in the prevalence of current health conditions, achieving on-site detection and quantifying serum albumin (SA) can contribute significantly to halting the increased mortality and morbidity rate. Herein, we have rationally designed and synthesized far-red emitting, solvatofluorochromic styryl chromone (SC) derivatives SC1 and SC2, and SC2-conjugated fluorescent magnetic nanoparticles (SCNPs) for sensing SA with a fluorogenic response via interacting at an atypical drug binding site. In solution, the highly sensitive and selective fluorogenic response was evaluated by the prominent amplification and blue-shift in the emission maxima of the probes from deep red to dark yellow through an intermediate orange emission. The transformation of the fluorogen into a fluorophore was manifested through spectroscopic measurements. The stabilization of the probes at protein pockets was ascribed to the non-covalent interactions, such as H-bonding, cation-π, and hydrophobic interactions, as unveiled by docking studies. The practical applications revealed the novelty of SC derivatives through (a) the capability to detect SA isolated from real blood samples via a turn-on fluorescence response; (b) the design of a simple, cheap, and portable test-strip using a glass-slide loaded with solid-state emissive SC2, which provided differential emission color of the SC2-HSA complex in solution and the solid-state with increasing concentration of HSA. Moreover, a smartphone-based color analysis application was employed to obtain the ratio of green and red (G/R) channels, which was utilized for quantitative detection of HSA; (c) the biocompatibility of the SC1 was ascertained through confocal laser scanning microscopic imaging (CLSM). Detailed investigation showed that SC1 could entirely localize in the mitochondria and evolve as a promising biomarker for distinguishing cancer cells from normal cells. Additionally, the validation of uncommon binding of SC1 and SC2 between domains I and III was determined using competition experiments with a known site-specific binder and molecular docking studies. This unique property of the probes can be further exploited to understand the cellular intake of HSA-drug complexes in the multifaceted biological system. These results find the utility of SC derivatives as small molecule-based chemosensors for at-home SA detection and as a biomarker for cancer.

A near-infrared dicyanoisophorone-based fluorescent probe for discriminating HSA from BSA

Despite the rapid development of fluorescent probe techniques for the detection of human serum albumin (HSA), a probe that discriminates between HSA and bovine serum albumin (BSA) is still a challenging task, since their similar chemical structures. As a continuation of our work, herein, a dicyanoisophorone-based fluorescent probe DCO2 is systematically studied for discrimination of HSA from BSA. The photophysical and sensing performances of DCO2, including basic spectroscopic properties, sensing sensitivity, and selectivity, exhibits that DCO2 could selectively bind with HSA and display remarkable fluorescence enhancement (∼254-fold) at 685 nm. The gap of the fluorescent response of DCO2 between HSA and BSA is an obvious increase from 21% to 73% compared to the previous probe DCO1. The sensing mechanism was elucidated by Job's plot, displacement experiment, and molecular docking, suggesting that the specific response to HSA originated from the rigid donor structure and steric hindrance. DCO2 could be buried in the DS1 pocket of HSA, and only partly wedged into the DS1 pocket of BSA with exposing twisted N,N-diethylamino group outside. Application studies indicated that DCO2 has well detective behavior for HSA in the biological fluids. This work could provide a new approach to design HSA-specific near-infrared fluorescence probes.

Near-infrared emissive cyanine probes for selective visualization of the physiological and pathophysiological modulation of albumin levels

With the promising advantages of the near-infrared region (NIR) emissive markers for serum albumin becoming very prominent recently, we devised CyG-NHS as the cyanine derived longest NIR-I emissive optical marker possessing albumin selective recognition ability in diverse biological milieu. Multiscale modeling involving molecular docking, molecular dynamics, and implicit solvent binding free energy calculations have been employed to gain insights into the unique binding ability of the developed probe at domain-I of albumin, in contrast to the good number of domain IIA or IIIA binding probes available in the literature reports. The binding free energy was found to be -31.8 kcal mol^-1 majorly predominated by hydrophobic interactions. Besides, the conformational dynamics of CyG-NHS in an aqueous medium and the albumin microenvironment have been comprehensively studied and discussed. The potentiality of this optical platform to monitor the intracellular albumin levels in human hepatoma (HepG2) cells in different pathophysiological states has been demonstrated here. Also, the competency of the phenformin drug in restoring the albumin levels in chronic hyperinsulinemic and hypercholesterolemic in vitro models has been established through the visualization approach. Altogether, the findings of this study throw light on the significance of the development of a suitable optical marker for the visualization of critical bioevents related to albumin.

Development of a human serum albumin structure-based fluorescent probe for bioimaging in living cells

Forming a stable complex is a prerequisite for intramolecular charge transfer (ICT) probe to recognize proteins. Herein, a human serum albumin (HSA) structure-based fluorescent probe DNPM was fabricated successfully with fully considering its binding to the primary sites in HSA. Molecular simulation was used to assist the probe design. Two ICT ligands DNPM and MPM were initially designed. Both DNPM and MPM had favorable HSA binding abilities, but only DNPM had a satisfactory HSA sensitivity. Electromagnetic coupling played a key role in DNPM fluorescence enhancement. Due to the electromagnetic environment difference in protein structure, DNPM only exhibited strong sensitivity to serum albumins. DNPM could bind to Sudlow site I and site II in HSA but could not be displaced from its binding sites by common site specific drugs (e.g. phenylbutazone and ibuprofen). Besides, DNPM exhibited great potential for illumining serum albumin in living cells. The results provided a beneficial approach for designing and synthesizing high sensitive and selective fluorescent probes for proteins.

Modulating donor of dicyanoisophorone-based fluorophores to detect human serum albumin with NIR fluorescence

It is urgently needed to develop NIR-fluorescent probe for detection of human serum albumin (HSA) since the interference of short-wavelength-fluorescence from endogenous species in real serum and urine. However, most previous reports were located in the short-wavelength region (<600 nm). In this work, a series of dicyanoisophorone (DCO)-based fluorophores 1-4 with different donor groups have been designed and investigated. A systematic study of their photophysical properties has been carried out. Among these probes, 4 exhibited NIR emission with the highest fluorescence brightness and the most sensitive signal response to HSA. Further studies demonstrated that 4 could strongly bind into the DS1 pocket of HSA with a 1:1 ratio. Importantly, the method based on 4 has been proven to be capable of sensing HSA in real serum and urine samples.

Development of an enhanced immunoassay based on protein nanoparticles displaying an IgG-binding domain and luciferase

Detection of small amounts of target molecules with high sensitivity is important for the diagnosis of many diseases, including cancers, and is particularly important to detect early stages of disease. Here, we report the development of a temperature-responsive fusion protein (ELP-DCN) comprised of an elastin-like polypeptide (ELP), poly-aspartic acid (D), antibody-binding domain C (C), and NanoLuc luciferase (N). ELP-DCN proteins form nanoparticles above a certain threshold temperature that display an antibody-binding domain and NanoLuc luciferase on their surface. ELP-DCN nanoparticles can be applied for enhancement of immunoassay systems because they provide more antibody-binding sites and an increased number of luciferase molecules, resulting in an increase in assay signal. Here, we report the detection of human serum albumin (HSA) as a model protein using anti-HSA and ELP-DCN proteins. Upon formation of ELP-DCN nanoparticles, the detection limit improved tenfold compared to the monomeric form of ELP-DCN.