Red emitting human serum albumin templated copper nanoclusters as effective candidates for highly specific biosensing of bilirubin

Biosynthesis of copper nanoclusters for fluorescence detection of bilirubin in biofluids

Copper nanoclusters (Cu NCs) have shown significant attention in sensing of molecular and ionic species. In this work, a single-step biosynthetic approach was introduced for the preparation of fluorescent Cu NCs using Holarrhena pubescens (H. pubescens) leaves extract as a template. The synthesized H. pubescens-Cu NCs act as a nanomolecular probe for the detection of bilirubin in biofluids. The synthesized H. pubescens-Cu NCs displayed highest fluorescence intensity at 454 nm, when excited at 330 nm. Importantly, selective detection of bilirubin was obtained by introducing H. pubescens-Cu NCs as a simple molecular probe. The interaction of bilirubin and H. pubescens-Cu NCs resulted in a remarkable decrease in the emission peak intensity. The developed H. pubescens-Cu NCs-based bilirubin molecular probe has a wide linear range of 0.5-20.00 μM with the limit of detection of 30.54 nM for bilirubin. The promising application of H. pubescens-Cu NCs-based molecular probe was assessed by assaying bilirubin in spiked biofluids.

Fluorescent Nitrogen-doped Carbon Dots-based Turn-off Sensor for Bilirubin

Bilirubin (BR), a heme protein produced from breakdown of haemoglobin is present in aged red blood cells; whose abnormal concentration is associated with diseases like hyperbilirubinemia, coronary disease, iron deficiency, and so on. Herein, we have synthesized a selective, sensitive, and low-cost sensing platform using fluorescent nitrogen doped carbon dots (NCDs), prepared from precursors; citric acid and urea via a simple microwave-assisted method. The emission at 444 nm on excitation with 360 nm was well quenched in presence of BR suggesting a direct turn-off detection for BR. Characterization of developed probe was done by UV-Visible absorption studies, photoluminescence studies, SEM, TEM, ATR-FTIR, XPS, and DLS analysis. BR was detected with a Limit of Detection (LoD) and Limit of Quantification (LoQ) of 0.32 µM and 1.08 µM respectively. NCDs exhibited excellent selectivity and sensitivity towards BR in the presence of co-existing biomolecules and ions. Practical feasibility was checked by paper-strip-based sensing of BR and spiked real human samples were used for conducting real sample analysis.

Orange-Red-Emitting Carbon Dots for Bilirubin Detection and Its Antibacterial Activity Against Escherichia coli and Staphylococcus aureus

In this study, orange-red-emitting carbon dots (OR-CDs) were prepared from p-phenylenediamine (p-PDA) and urea as starting precursors through the hydrothermal method. The OR-CDs exhibited bright orange-red fluorescence at 618 nm when excited at 480 nm. The obtained OR-CDs exhibited stable photophysical properties under different physiological conditions. The unique photophysical property of OR-CDs were then utilized for fluorometric determination of bilirubin. The fluorometric assay revealed that the fluorescence intensity of OR-CDs is gradually quenched upon the addition of bilirubin (1-20 μM). The mechanism of fluorescence quenching was evaluated by steady-state fluorescence analysis and time-correlated single photon counting measurements. The OR-CDs showed good selectivity and sensitivity toward bilirubin over other common interfering biomolecules. The present fluorometric assay showed a linear response toward bilirubin between 1 and 10 μM with a limit of detection of 4.80 nM. Further, a fluorescence test cotton swab-based detection probe has been successfully developed by incorporating OR-CDs for the point-of-care detection of bilirubin in biofluids. Furthermore, a light-emitting diode light that emits orange-red light was prepared by embedding the OR-CDs within the poly(vinyl alcohol) polymer matrix. Moreover, the antibacterial activity of OR-CDs was tested against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus. The antibacterial efficacy of OR-CDs was demonstrated by various mechanisms, such as reactive oxygen species generation, destruction of cell structure, chemical binding to membrane, and surface wrapping. Interestingly, the survival assay against L929 fibroblast cells exhibits favorable biocompatibility and bioimaging.

Label-Free Detection of Unbound Bilirubin and Nitrophenol Explosives in Water by a Mechanosynthesized Dual Functional Zinc Complex: Recognition of Picric Acid in Various Common Organic Media

High levels of unconjugated bilirubin (UB) in serum lead to asymptomatic and neonatal jaundice and brain dysfunctions. Herein, we have reported the detection of UB at as low as 1 μM in an aqueous alkaline medium using a Zn(II) complex. The specificity of the complex has been validated by the HPLC in the concentration window 6-90 μM, which is rare. The sensory response of the probe at physiological pH against nitro explosives developed it as an instant-acting fluorosensor for picric acid (PA) and 2,4-dinitrophenol (2,4-DNP). Spectroscopic titration provided a binding constant of 4×105  M-1 with PA. The naked eye detection was found to be 15 μM. The solid-state photoluminescent nature of the complex enabled it for PA sensing in the solid phase. Interestingly, the probe remained fluorescent in various volatile and non-volatile organic solvents. As a result, it can also detect PA and 2,4-DNP in a wide range of common organic media. NMR studies revealed the coordination of PA, 2,4-DNP, and UB to the Zn(II) center of the probe, which is responsible for the observed quenching of the probe with the analytes.

Protein-Templated Metal Nanoclusters: Molecular-like Hybrids for Biosensing, Diagnostics and Pharmaceutics

The use of proteins as biomolecular templates to synthesize atomically precise metal nanoclusters has been gaining traction due to their appealing properties such as photoluminescence, good colloidal- and photostability and biocompatibility. The synergistic effect of using a protein scaffold and metal nanoclusters makes it especially attractive for biomedical applications. Unlike other reviews, we focus on proteins in general as the protective ligand for various metal nanoclusters and highlight their applications in the biomedical field. We first introduce the approaches and underlined principles in synthesizing protein-templated metal nanoclusters and summarize some of the typical proteins that have been used thus far. Afterwards, we highlight the key physicochemical properties and the characterization techniques commonly used for the size, structure and optical properties of protein-templated metal nanoclusters. We feature two case studies to illustrate the importance of combining these characterization techniques to elucidate the formation process of protein-templated metal nanoclusters. Lastly, we highlight the promising applications of protein-templated metal nanoclusters in three areas-biosensing, diagnostics and therapeutics.

Perspectives of different colour-emissive nanomaterials in fluorescent ink, LEDs, cell imaging, and sensing of various analytes

In the past 2 decades, multicolour light-emissive nanomaterials have gained significant interest in chemical and biological sciences because of their unique optical properties. These materials have drawn much attention due to their unique characteristics towards various application fields. The development of novel nanomaterials has become the pinpoint for different application areas. In this review, the recent progress in the area of multicolour-emissive nanomaterials is summarized. The different emissions (white, orange, green, red, blue, and multicolour) of nanostructure materials (metal nanoclusters, quantum dots, carbon dots, and rare earth-based nanomaterials) are briefly discussed. The potential applications of different colour-emissive nanomaterials in the development of fluorescent inks, light-emitting diodes, cell imaging, and sensing devices are briefly summarized. Finally, the future perspectives of multicolour-emissive nanomaterials are discussed.

Nanomaterials for fluorescent assay of bilirubin

The accumulation of bilirubin in blood is associated with many diseases. Sensitive and accurate detection of bilirubin is of great significance for personal health care. The rapid development of fluorescent nanomaterials promotes rapid development in the bilirubin assay. In this review, traditional methods for detection of bilirubin are briefly presented to compare with fluorescent nanosensors. Subsequently, the recent progress of different types of fluorescent nanomaterials for determination of bilirubin is summarized. Further, the performance of fluorescent nanosensors and conventional techniques for sensing bilirubin are compared. To this end, the challenges and prospects concerning the topics are discussed. This review will provide some introductory knowledge for researchers to understand the status and importance of fluorescent nanosensors for sensing bilirubin.

Novel disulfide bond bridged 7-ethyl-10-hydroxyl camptothecin-undecanoic acid conjugate/human serum albumin nanoparticles for breast cancer therapy

7-Ethyl-10-hydroxyl camptothecin (SN38), a semisynthetic derivative of camptothecin, exhibited extreme pharmacological activities in treating a range of cancers. However, its poor aqueous solubility and low stability hinder its clinical applications. Hence, a redox-responsive SN38 prodrug encapsulated human serum albumin (HSA) nanoparticle is developed to realize its potential in the clinic. First, a disulfide bond bridged 7-ethyl-10-hydroxyl camptothecin-undecanoic acid conjugate (SN38-SS-COOH) was synthesized and characterized structurally. After that, SN38-SS-COOH/HSA nanoparticles (SNH NPs) were prepared by the desolvation method. The SNH NPs with a feed molar ratio of 9 : 1 of SN38-SS-COOH : HSA showed a spherical structure with a diameter range of approximately 120-150 nm revealed by dynamic light scattering (DLS) and transmission electron microscopy (TEM). Fluorescence quenching confirmed the formation of SNH NP complexes by dual hydrophobic force and electrostatic interaction. The SNH NPs have a high drug loading of 10.44% and an encapsulation efficiency of 89.59% with good stability. Moreover, the redox responsiveness was validated by glutathione (GSH)-triggered accelerated release of parent drug SN38. In an in vivo pharmacokinetic study, the SNH NPs exhibited a significantly prolonged circulation time (t_1/2, 3.77-fold) compared with free SN38. Finally, the in vivo antitumor efficacy and systemic toxicity of SNH NPs in a breast xenograft model were thoroughly evaluated. The inhibition rate of tumor growth induced by the SNH NPs reached 70.1%, while only 50.1% was achieved for irinotecan at an equivalent SN38 dosage of 10 mg kg^-1. More importantly, the SNH NPs achieved a higher level of tumor growth inhibition (85.3%) by increasing the dosage to 60 mg kg^-1 SN38 without obvious adverse effects. Taken together, the use of redox-responsive SN38 prodrug/HSA NPs could be a promising strategy to deliver highly active SN38 for breast cancer chemotherapy.

A paper-based point-of-care testing device for the colourimetric estimation of bilirubin in blood sample

A paper-based colourimetric assay for the Point-of-Care Testing (PoCT) of bilirubin has been developed based on the formation of a green-coloured copper-bilirubin complex from a blue-coloured tetraamminecopper(II) sulphate complex. The reaction was studied and optimized by UV-Visible absorption spectroscopy and translated onto a paper strip. Hydrophobic circular well patterns on Whatman chromatography paper were created by wax printing. The tetraamminecopper(II) sulphate complex was drop cast and dried on the reagent zones in the wax-patterned paper. The images of reagent zones captured using a scanner were analyzed using ImageJ software. Bilirubin spiked blood serum was tested in the concentration range of 1.2 to 950 µM. The PAD exhibited sensitivities of 0.4197 a.u/μM and 0.1040 a.u/μM for concentration ranges of bilirubin 1.2 to 96 μM and 105 to 950 μM respectively and a low detection limit of 0.799 µM. The method is highly selective to bilirubin, even in the presence of other biomarkers in serum. A plasma separation membrane incorporated PAD was fabricated for the final testing and quantification of bilirubin from whole blood.

Synthesis of Copper Nanoclusters and Their Application for Environmental Pollutant Probes: A Review

Copper nanoclusters (CuNCs) as a new type of probe for environmental contaminants are gaining increasing attention because of its low cost, superior water dispersibility, wide availability and excellent optical properties. Compared with the other probes such as quantum dots and organic dyes, CuNCs show much more potential in practical application for their excellent photostability, large Stokes shift, low toxicity and other preponderance, especially in the fields of biosensing and environmental monitoring. Recently, the template-assisted synthesis of metal nanoclusters (MNCs) has been widely studied. A variety of templates such as proteins, small thiol molecules, polymers, and DNA with different spatial configuration have been used for the preparation of MNCs so far. This review primarily described recent advances in CuNCs in terms of the synthesis of CuNCs from different templates, the methods to improve the fluorescence (FL) properties of CuNCs, as well as the basic detection mechanisms based on the FL properties or catalytic properties. Finally, to promote the practical application of CuNCs probes, the challenges and prospects of CuNCs multifunctional probes are also discussed.

Engineering colloidally stable, highly fluorescent and nontoxic Cu nanoclusters via reaction parameter optimization

Metal nanoclusters (NCs) composed of the least number of atoms (a few to tens) have become very attractive for their emerging properties owing to their ultrasmall size. Preparing copper nanoclusters (Cu NCs) in an aqueous medium with high emission properties, strong colloidal stability, and low toxicity has been a long-standing challenge. Although Cu NCs are earth-abundant and inexpensive, they have been comparatively less explored due to their various limitations, such as ease of surface oxidation, poor colloidal stability, and high toxicity. To overcome these constraints, we established a facile synthetic route by optimizing the reaction parameters, especially altering the effective concentration of the reducing agent, to influence their optical characteristics. The improvement of the photoluminescence intensity and superior colloidal stability was modeled from a theoretical standpoint. Moreover, the as-synthesized Cu NCs showed a significant reduction of toxicity in both in vitro and in vivo models. The possibility of using such Cu NCs as a diagnostic probe toward C. elegans was explored. Also, the extension of our approach toward improving the photoluminescence intensity of the Cu NCs on other ligand systems was demonstrated.

Controlling the Chemistry of Nanoclusters: From Atomic Precision to Controlled Assembly

Metal nanoclusters have gained prominence in nanomaterials sciences, owing to their atomic precision, structural regularity, and unique chemical composition. Additionally, the ligands stabilizing the clusters provide great opportunities for linking the clusters in higher order dimensions, eventually leading to the formation of a repertoire of nanoarchitectures. This makes the chemistry of atomic clusters worth exploring. In this mini review, we aim to focus on the chemistry of nanoclusters. Firstly, we summarize the important strategies developed so far for the synthesis of atomic clusters. For each synthetic strategy, we highlight the chemistry governing the formation of nanoclusters. Next, we discuss the key techniques in the purification and separation of nanoclusters, as the chemical purity of clusters is deemed important for their further chemical processing. Thereafter which we provide an account of the chemical reactions of nanoclusters. Then, we summarize the chemical routes to the spatial organization of atomic clusters, highlighting the importance of assembly formation from an application point of view. Finally, we raise some fundamentally important questions with regard to the chemistry of atomic clusters, which, if addressed, may broaden the scope of research pertaining to atomic clusters.

Highly fluorescent carbon dots from coix seed for the determination of furazolidone and temperature

In this work, blue emission fluorescent carbon dots (CDs) were fabricated by using the hydrothermal strategy from coix seed for the first time. We found that the prepared CDs possessed many excellent characteristics including excitation-dependent properties, good solubility and strong photostability. The optimal excitation and emission wavelength of CDs were 363 and 435 nm, respectively. Unbelievably, the fluorescence of CDs was selectively and effectively quenched with the addition of furazolidone (Fu). The quenching mechanisms might be assumed to the static quenching and inner filter effect (IFE). Based on this principle, a novel fluorescence probe was developed for the determination of Fu. At the same time, the proposed probe showed excellent sensitivity and selectivity towards Fu with a wide linear range from 0.5 to 100 μM, and the corresponding detection limit was 0.096 μM. Moreover, the CDs also could be applied for the sensing of temperature. The practical application of the CDs for Fu detection in real samples was also confirmed with the satisfactory recoveries changing from 96.6% to 108.5%, which provided huge possibility in the field of medical analysis.

Highly sensitive and selective fluorescence sensing of nitrofurantoin based on water-soluble copper nanoclusters

In this contribution, dopamine-protected copper nanoclusters as a novel fluorescent nanosensor was employed to detect nitrofurantoin (NFT) for the first time, which were prepared by using dopamine as the stabilizing agent and sodium borohydride (NaBH₄) and hydrazine hydrate (N₂H₄·H₂O) as the reducing agents. A series of methods were used to analyze the structure and optical properties of as-prepared Cu NCs, such as UV-Vis absorption spectroscopy, fluorescence spectroscopy, Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It possessed high dispersion, excellent water solubility, good photostability and strong green fluorescence under UV lamp (365 nm). Significantly, the FL intensities of Cu NCs were quenched with addition of NFT. The analytical method possessed good linear relationship between the relative fluorescence intensity (F₀/F) and the NFT concentrations (range from 5 to 120 μM), and the limit of detection (LOD) could reach 0.73 μM. The fluorescence detection mechanisms were attributed to the static quenching and inner filter effect (IFE). In addition, this proposed fluorescence sensor has been successfully used for the detection of NFT in bovine serum samples.

Investigation on the luminescence behavior of terbium acetylsalicylate/bilirubin system via 2D-COS approaches

Terbium acetylsalicylate has been prepared, and the ethanol solution of the complex exhibits strong luminescence under the excitation of ultraviolet radiation. When a small amount of bilirubin solution is introduced into the solution containing a high concentration of terbium acetylsalicylate, a remarkable diminution of the luminescence of the terbium complex was observed. Investigations on the behavior and life-time of luminescence indicate that the quenching is not caused by forming a stable non-luminescent product via a reaction between terbium acetylsalicylate and bilirubin. A π-π interaction between the chromophore of bilirubin and the aromatic moiety of ligand was revealed via the pattern of cross peaks in the 2D asynchronous spectrum generated using the DAOSD (double asynchronous orthogonal sample design) approach. Such an interaction paves a route for energy transfer in the quenching process. The combination of a high concentration of the terbium complex and a long life-time of luminescence in the lanthanide complex/bilirubin system forms a special scenario: a bilirubin molecule by diffusion may visit and deactivate dozens of excited terbium complexes within the half-life period of the lanthanide complex. This is why a small amount of bilirubin can bring about the significant reduction of luminescence on the solution containing a high concentration of the terbium complex.

Copper nanoclusters: designed synthesis, structural diversity, and multiplatform applications

Atomically precise metal nanoclusters (MNCs) have gained tremendous research interest in recent years due to their extraordinary properties. The molecular-like properties that originate from the quantized electronic states provide novel opportunities for the construction of unique nanomaterials possessing rich molecular-like absorption, luminescence, and magnetic properties. The field of monolayer-protected metal nanoclusters, especially copper, with well-defined molecular structures and compositions, is relatively new, about two to three decades old. Nevertheless, the massive progress in the field illustrates the importance of such nanoobjects as promising materials for various applications. In this respect, nanocluster-based catalysts have become very popular, showing high efficiencies and activities for the catalytic conversion of chemical compounds. Biomedical applications of clusters are an active research field aimed at finding better fluorescent contrast agents, therapeutic pharmaceuticals for the treatment and prevention of diseases, the early diagnosis of cancers and other potent diseases, especially at early stages. A huge library of structures and the compositions of copper nanoclusters (CuNCs) with atomic precisions have already been discovered during last few decades; however, there are many concerns to be addressed and questions to be answered. Hopefully, in future, with the combined efforts of material scientists, inorganic chemists, and computational scientists, a thorough understanding of the unique molecular-like properties of metal nanoclusters will be achieved. This, on the other hand, will allow the interdisciplinary researchers to design novel catalysts, biosensors, or therapeutic agents using highly structured, atomically precise, and stable CuNCs. Thus, we hope this review will guide the reader through the field of CuNCs, while discussing the main achievements and improvements, along with challenges and drawbacks that one needs to face and overcome.

Protein-templated copper nanoclusters for fluorimetric determination of human serum albumin

Copper nanoclusters (CuNCs) are attractive for their unique optical properties, providing sensitive fluorescent detection of several kinds of targets even in complex matrices. Their ability in growing on suitable protein and nucleic acid templates make CuNCs efficient optical reporters to be exploited in bioanalysis. In this work, we report the specific and sensitive determination of human serum albumin (HSA) in human serum (HS) and urine via CuNCs fluorescence. HSA is the most abundant protein in plasma, and plays a key role in the early diagnosis of serious pathological conditions such as albuminuria and albuminemia. Recently, HSA has become clinically central also as a biomarker to assess severity, progression, and prognosis of various cancers. We report the controlled and reproducible growth of CuNCs directly on the target analyte, HSA, which results in a fine dose-dependent fluorescent emission at 405 nm. The protocol is optimized in water, and then applied to serum and urine specimens, without matrix pretreatment. The method linearly responds within the whole concentration of clinical interest, with a sensitivity of 1.8 ± 0.1 × 10^-3 g L^-1 and 0.62 ± 0.03 × 10^-3 g L^-1 in serum and urine, respectively, and excellent reproducibility (CV_av% ca. 3% for both). The assay is designed to have a single protocol working for both matrices, with recovery of 95% (HS) and 96% (urine). The stability of the fluorescence after CuNCs formation was tested over 3 days, displaying good results (yet higher in urine than in serum).

Aggregation-induced emission enhancement of gold nanoclusters in metal-organic frameworks for highly sensitive fluorescent detection of bilirubin

A fluorescence analysis method based on gold nanocluster (AuNC) and metal-organic framework (MOF) composite materials (AuNCs@ZIF-8) was established for highly sensitive detection of bilirubin (BR). First, AuNCs@ZIF-8 was successfully obtained by co-precipitation and displayed an aggregation-induced emission enhancement by the confinement effect of the MOFs (i.e., ZIF-8). The product showed approximately 7.0 times enhancement in the quantum yield and longer fluorescence lifetime from 2.29 μs to 11.51 μs compared with AuNCs. When BR combined with the metal node Zn2+ of ZIF-8, the skeleton of the composite was destroyed, leading to a great decrease in the fluorescence intensity by the transformation of the AuNCs from the aggregated state to dispersed state. The linear range for the detection of BR was 0.1-5.0 μM, with the limit of detection (LOD) of 0.07 μM (S/N = 3). The AuNCs@ZIF-8 exhibited a selective response toward BR within 5 min and detected BR in human serum. The long-wavelength emission by AuNCs avoided the interference of the complex biomatrix background fluorescence, indicating their great application prospects for clinical diagnosis.

Smart phone assisted quinoline-hemicyanine based fluorescent probe for the selective detection of glutathione and the application in living cells

Quinoline appended hemicyanine 6MIM with strong ICT character was successfully synthesized through simple condensation reaction of 6-methoxy-2-chloro-3-formyl quinoline with 2-benzothiazolinium iodide. The photophysical characteristics of synthesized probe revealed that it would selectively detect glutathione (GSH) when it compared with different amino acids including biothiols and the detection limit is found to be 100 nM. The turn off sensor is due to thiol-halogen S_NAr nucleophilic substitution between 6MIM and thiol group in glutathione. More importantly, the biosensor 6MIM was effectively applied in the fluorescence bioimaging of GSH in living cells with low cell toxicity. The colorimetric detectable color change of 6MIM-GSH has been effectively integrated with smartphone assisted RGB color value application with lowest detection value of 120 nM.

Purification and separation of ultra-small metal nanoclusters

Metal nanoclusters (NCs) are ultra-small nanoparticles intermediate in size between small molecule complexes and nanoparticles. NCs with tunable surface functionality feature unique physical and chemical properties, however these properties are frequently compromised by the presence of undesired components such as excess ligands or mixtures of NCs. In a typical synthesis process, different NCs can be formed with varying numbers of metal atoms and/or ligands, and even NCs with the same number of metal atoms and ligands can have different spatial structures. The separation of pure NCs is important because different species have distinct optical and catalytic behavior. However, NCs can be difficult to purify or separate for a range of reasons. In this review, we discuss established and emerging approaches for NC purification/separation, with a focus on choosing the appropriate method depending on NC and application.