Fluorescent DNA-Silver nanoclusters in food safety detection: From synthesis to application

In recent years, the conventional preparation of silver nanoclusters (AgNCs) has attracted much attention due to their ultra-small size, tunable fluorescence, easy-to-engineer, as well as biocompatible material. Moreover, its great affinity towards cytosine bases on single-stranded DNA has led to the construction of biosensors, especially aptamers, for a broad variety of applications in food safety and environmental protection. In past years, numerous researchers paid attention to the construction of AgNCs aptasensor. Therefore, this review will be an effort to summarize the synthetic strategy along with the influences of factors on synthesis, categorize the sensing mechanism of aptamer-functionalized AgNCs biosensors, as well as their specific applications in food safety detection including heavy metal, toxin, and foodborne pathogenic bacteria. Furthermore, a brief conclusion and outlook regarding the prospects and challenges of their applications in food safety were drawn in line with the developments in DNA-AgNCs.

Nanohybrids of atomically precise metal nanoclusters

Atomically precise metal nanoclusters (NCs) with molecule-like structures are emerging nanomaterials with fascinating chemical and physical properties. Photoluminescence (PL), catalysis, sensing, etc., are some of the most intriguing and promising properties of NCs, making the metal NCs potentially beneficial in different applications. However, long-term instability under ambient conditions is often considered the primary barrier to translational research in the relevant application fields. Creating nanohybrids between such atomically precise NCs and other stable nanomaterials (0, 1, 2, or 3D) can help expand their applicability. Many such recently reported nanohybrids have gained promising attention as a new class of materials in the application field, exhibiting better stability and exciting properties of interest. This perspective highlights such nanohybrids and briefly explains their exciting properties. These hybrids are categorized based on the interactions between the NCs and other materials, such as metal-ligand covalent interactions, hydrogen-bonding, host-guest, hydrophobic, and electrostatic interactions during the formation of nanohybrids. This perspective will also capture some of the new possibilities with such nanohybrids.

DNA-Guided Metallization of Nanomaterials and Their Biomedical Applications

Precise control of the structure of metallic nanomaterials is critical for the advancement of nanobiotechnology. As DNA (deoxyribonucleic acid) can readily modify various moieties, such as sulfhydryl, carboxyl, and amino groups, using DNA as a directing ligand to modulate the morphology of nanomaterials is a promising strategy. In this review, we focus on the use of DNA as a template to control the morphology of metallic nanoparticles and their biomedical applications, discuss the use of DNA for the metallization of gold and silver, explore the factors that influence the process, and outline its biomedical applications. This review aims to provide valuable insights into the DNA-guided growth of nanomaterials. The challenges and future directions are also discussed.

Recent advances in the visual detection of ions and molecules based on gold and silver nanoclusters

Gold and silver nanoclusters (Au/AgNCs) exhibit excellent application potential in optical biosensors because of their low toxicity, excellent biocompatibility, and unique optical properties. Au/AgNCs-based visual analysis methods have emerged as powerful tools for detecting various targets with convenient readout. In this review, the applications of Au/AgNCs in the visual detection and bioimaging of metal ions, inorganic anions, small molecules, and biomacromolecules in various devices are summarized. Furthermore, this review also discusses the future perspectives of the field.

Combining metal nanoclusters and carbon nanomaterials: Opportunities and challenges in advanced nanohybrids

The development of functional materials with uniquely advanced properties lies at the core of nanoscience and nanotechnology. From the myriad possible combinations of organic and/or inorganic blocks, hybrids combining metal nanoclusters and carbon nanomaterials have emerged as highly attractive colloidal materials for imaging, sensing (optical and electrochemical) and catalysis, among other applications. While the metal nanoclusters provide extraordinary luminescent and electronic properties, the carbon nanomaterials (of zero, one or two dimensions) convey versatility, as well as unique interfacial, electronic, thermal, optical, and mechanical properties, which altogether can be put to use for the desired application. Herein, we present an overview of the field, for experts and non-experts, encompassing the basic properties of the building blocks, a systematic view of the chemical preparation routes and physicochemical properties of the hybrids, and a critical analysis of their ongoing and emerging applications. Challenges and opportunities, including directions towards green chemistry approaches, are also discussed.

Graphene oxide/perylene-aniline electrochemiluminescence platform for protein detection based on molecule recognition

Most biosensors for folate receptor (FR) detection based on folic acid (FA) recognition usually contain FA-linked single-strand DNA (FA-ssDNA) and nuclease to promote sensitivity, which increases expenses and involves complicated assay processes. A few electrochemiluminescence (ECL) sensors that do not use FA-ssDNA and nuclease directly graft FA onto an ECL nanomaterial through covalent bonding for FR detection. In this study, we used FA-ssDNA to non-covalently graft FA onto π-conjugated ECL nanomaterial graphene oxide (GO)/perylene-aniline for fabricating ultrasensitive FR sensors without nuclease. 3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA) and aniline (An) self-assembled into π-conjugated nanorods, which were then loaded onto GO. This material was reported to produce 673 nm-dominated ECL with the co-reactant K₂S₂O₈, and was used as an ECL platform. FA-modified Poly-dA-ssDNA (FA-Poly-dA-ssDNA) molecules, consisting of 20 bases, were attached to the surface of GO/PTCDA-An to capture FR. A significant decrease of ECL intensity was observed due to the steric hindrance of FR. The proposed sensors exhibited high detection sensitivity with a linear range from 1 fg mL^-1 to 1 ng mL^-1 and a detection limit of 0.636 fg mL^-1. The sensors also showed good potential in real sample detection. Without introducing nuclease and complicated chemical reactions, this work provides a new sensing strategy for protein detection based on molecular recognition, which is extremely important in clinical diagnosis.

Diagnosis of cancer at early stages based on the multiplex detection of tumor markers using metal nanoclusters

Traditional cancer diagnosis strategies are not considered by most people until the last resort, which delays many cancer treatments leading to advanced stages. Tumor marker sensors show great potential for detecting cancer because of its cost-effective and harmless checking procedures. Normally, one tumor marker is detected each time by using one type of sensor, but the accuracy to declare cancer is not always satisfied. Metal nanoclusters are ultra-small nanomaterials with low toxicity, distinct optical properties, catalytic activities, and cost-effective performance. Some metal nanoclusters have been designed to detect more than one tumor marker in a single step. The consideration of combined parameters using such facile sensing strategies has the potential to simplify the test procedure, and increase the diagnostic accuracy of early cancer. Therefore, various sensing strategies for the multiplex detection of tumor markers using metal nanoclusters are summarized.

Direct Laser Writing of Fluorescent Silver Nanoclusters: A Review of Methods and Applications

Metal nanoclusters (NCs) are nanomaterials of size of less than 2 nm that exhibit a set of unique physical, chemical, optical, and electronic properties. Because of recent interest in NCs, a great deal of effort is being made to develop synthetic routes that allow control over the NC size, shape, geometry, and properties. Direct laser writing is one of the few synthesis methods that allow the generation of photostable NCs with high quantum yield in a highly controlled fashion. A key advantage of laser-written NCs is the ability to create easy-to-use solid-state devices for a range of applications. This review will present necessary background and recent advances in laser writing of silver NCs and their applications in different solid-state matrixes such as glass, zeolites, and polymer substrate. This topic will be of interest to researchers in the fields of materials science, optics and photonics, chemistry, and biomedical sciences.

Zinc-Ion-Induced Aggregation of Gold Clusters for Visible-Light-Excitation-Based Fluorimetric Discrimination of Geometrical Isomers

Herein, we report discrimination of dicarboxylic acids - fumaric acid (FA) and maleic acid (MA) - exhibiting geometrical isomerism, using nanoclusters based luminescent probe having excitation under broad day light. The luminescent probe was designed via complexation reaction between zinc ions and ligands (mercaptopropioinc acid; MPA) stabilizing the gold nanoclusters. This resulted in formation of nanoaggregates exhibiting bright green luminescence upon excitation at 450 nm capable of discriminating between FA and MA upto nanomolar level. The basis of discrimination has been attributed to deprotonation of FA and MA following interaction with MPA moieties present on the surface of the nanoaggregates and being governed by the stability of the respective conjugate base of the geometrical isomers of the dicarboxylic acids. As a consequence of different extent of deprotonation of FA and MA upon interaction with the cluster aggregates, different effect on the luminescence of the aggregates was observed, thus enabling discernible fluorimetric discrimination between FA and MA under visible light excitation.

Microfluidic-Based Immunomodulation of Immune Cells Using Upconversion Nanoparticles in Simulated Blood Vessel-Tumor System

The goal of cancer immunotherapy is the selective killing of malignant cells by the cooperated efforts of immune cells at the primary and secondary sites. Here, we developed folic acid and secondary lymphoid tissue chemokine-loaded mesoporous silica-modified upconversion nanoparticle construct as a targeting, delivery, and imaging system to attract immune cells to folate receptor-expressing tumor cells. The effectiveness of the nanoparticles in targeting dendritic cells and T cells to the tumor compartment was tested in a vasculature-tumor interface model constructed from the co-culture of endothelial cells and ovarian cancer cells, in different interconnected channels in a microfluidic device. In comparison to the unconjugated nanoparticles, the folic acid-conjugated nanoparticles efficiently diffuse across the engineered blood vessel and specifically target the folate receptor-expressing ovarian cancer cells. The developed microfluidic platform was further used to demonstrate increased dendritic cell and T cell migration toward the ovarian cancer cell channel induced by the presence of the chemokine- and folic acid-loaded nanoparticles. The nanoparticle construct did not exhibit any significant cyto- and hemotoxicity. This proof of concept showed the potential of the nanoparticles to target cancer cells as well as to recruit dendritic cells and T cells to tumor sites to augment the weak host immune response.

Integration of fluorescence imaging and electrochemical biosensing for both qualitative location and quantitative detection of cancer cells

In this work, DNA-templated silver nanoclusters (DNA-AgNCs) with unique fluorescent and electrochemical properties are prepared as dual signal probes for both qualitative imaging and quantitative detection of cancer cells in an integrated system. ITO electrode that has good light transmittance and electric conductivity is employed as a substrate for dual analysis of cancer cells. ITO electrode is firstly modified by AS1141 aptamer, which could selectively bind to nucleolin overexpressed on the surface of a model breast cancer cell, MCF-7 cell line. The composite of mucin 1 antibody (anti-MUC1) and DNA-AgNCs then binds to MUC1 on the surface of captured MCF-7 cell, forming a sandwich-like structure. Therefore, our method allows noninvasive fluorescence imaging and amplified electrochemical detection using a single labeling platform, providing a biocompatible and highly specific method for adequate analysis of cancer cells. Experimental results demonstrate that strong red fluorescence of DNA-AgNCs clearly displays the loading of cancer cells on ITO electrode after dual recognition, and amplified electrochemical signals of DNA-AgNCs enable improved sensitivity toward quantitative analysis with a detection limit of 3 cells.

Tailoring the photoluminescence of atomically precise nanoclusters

Due to their atomically precise structures and intriguing chemical/physical properties, metal nanoclusters are an emerging class of modular nanomaterials. Photo-luminescence (PL) is one of their most fascinating properties, due to the plethora of promising PL-based applications, such as chemical sensing, bio-imaging, cell labeling, phototherapy, drug delivery, and so on. However, the PL of most current nanoclusters is still unsatisfactory-the PL quantum yield (QY) is relatively low (generally lower than 20%), the emission lifetimes are generally in the nanosecond range, and the emitted color is always red (emission wavelengths of above 630 nm). To address these shortcomings, several strategies have been adopted, and are reviewed herein: capped-ligand engineering, metallic kernel alloying, aggregation-induced emission, self-assembly of nanocluster building blocks into cluster-based networks, and adjustments on external environment factors. We further review promising applications of these fluorescent nanoclusters, with particular focus on their potential to impact the fields of chemical sensing, bio-imaging, and bio-labeling. Finally, scope for improvements and future perspectives of these novel nanomaterials are highlighted as well. Our intended audience is the broader scientific community interested in the fluorescence of metal nanoclusters, and our review hopefully opens up new horizons for these scientists to manipulate PL properties of nanoclusters. This review is based on publications available up to December 2018.

In situ fluorescence activation of DNA-silver nanoclusters as a label-free and general strategy for cell nucleus imaging

A facile, general and turn-on nucleus imaging strategy was first developed based on in situ fluorescence activation of C-rich dark silver nanoclusters by G-rich telomeres. After a simple incubation without washing, nanoclusters could selectively stain the nucleus with intense red luminescence, which was confirmed using fixed/living cells and several cell lines.

A cationic conjugated polymer coupled with exonuclease I: application to the fluorometric determination of protein and cell imaging

A strategy is described for the detection of protein by using a cationic fluorescent conjugated polymer coupled with exonuclease I (Exo I). Taking streptavidin (SA) as model protein, it is observed that Exo I can digest single-stranded DNA conjugated with biotin and carboxyfluorescein (P1) if SA is absent. This leads to the formation of small nucleotide fragments and to weak fluorescence resonance energy transfer (FRET) from the polymer to P1. If, however, SA is present, the high affinity of SA and biotin prevents the digestion of P1 by Exo I. This results in the sorption of P1 on the surface of the polymer through strong electrostatic interaction. Hence, efficient FRET occurs from the fluorescent polymer to the fluorescent label of P1. Fluorescence is measured at an excitation wavelength of 370 nm, and emission is measured at two wavelengths (530 and 425 nm). The ratio of the two intensities (I₅₃₀/I₄₂₅) is directly related to the concentration of SA. Under the optimal conditions, the assay has a detection limit of 1.3 ng·mL^-1. The method was also applied to image the folate receptor in HeLa cells, thus demonstrating the versatility of this strategy. Graphical abstract A fluorometric strategy is described for protein detection and cell imaging based on a cationic conjugated polymer (PFP) coupled with exonuclease I (Exo I) trigged fluorescence resonance energy transfer (FRET).

DNA metallization: principles, methods, structures, and applications

This review summarizes the research activities on DNA metallization since the concept was first proposed in 1998, covering the principles, methods, structures, and applications.

A turn-on fluorescence assay of alkaline phosphatase activity using a DNA–silver nanocluster probe

A label-free fluorescence assay has been developed for the detection of alkaline phosphatase based on DNA–silver nanocluster probes.

Ratiometric Catalyzed-Assembly of NanoCluster Beacons: A Nonenzymatic Approach for Amplified DNA Detection

In this work, a novel fluorescent transformation phenomenon of oligonucleotide-encapsulated silver nanoclusters (AgNCs) was demonstrated, in which green-emissive AgNCs effectively transformed to red-emissive AgNCs when placed in close proximity to a special DNA fragment (denoted as convertor here). Taking advantage of a catalyzed-hairpin-assembly (CHA) amplification strategy, we rationally and compatibly engineered a simple and sensitive AgNC-based fluorescent signal amplification strategy through the ratiometric catalyzed-assembly (RCA) of green-emissive NanoCluster Beacon (NCB) with a convertor modified DNA hairpin to induce the template transformation circularly. The proposed ratiometric fluorescent biosensing platform based on RCA-amplified NCB (RCA-NCB) emits intense green fluorescence in the absence of target DNA and will undergo consecutively fluorescent signal transformation from green emission to red emission upon exposure to its target DNA. The ratiometric adaptation of the NCB to CHA circuit advances their general usability as biosensing platform with great improvements in detection sensitivity. By measuring the fluorescence intensity ratio of the red emission and green emission, the proposed RCA-NCB platform exhibits sensitive and accurate analytical performance toward Werner Syndrome-relevant gene, the proof-of-concept target in this work. A low detection limit down to the pM level was achieved, which is lower than most of the reported AgNC-based fluorescent DNA biosensors, making the proposed RCA-NCB biosensing strategy appealing in amplifying the ratiometric fluorescent signal for sensitive DNA detection. Moreover, our proposed RCA-NCB platform shows good recovery toward the target DNA in real human serum samples, illustrating their potential promise for clinical and imaging applications in the future.

Ratiometric NanoCluster Beacon: A Label-Free and Sensitive Fluorescent DNA Detection Platform

Although researches until now have emphasized the influence of an oligonucleotide sequence on the fluorescence of oligonucleotide-stabilized silver nanoclusters (AgNCs), this influence has been explored as a novel ratiometric fluorescent signal transduction in this work. This study builds on our original discovery of a template-transformation phenomenon, which demonstrated that the connection of a special DNA fragment (5'-CACCGCTTT-3') with a green-emitting AgNC nucleation sequence (GNuS, 5'-TGCCTTTTGGGGACGGATA-3') creates a red-emitting AgNC nucleation sequence (RNuS, 5'-CACCGCTTTTGCCTTTTGGGGACGGATA-3'). Attempts to expand this idea and construct elegant ratiometric NanoCluster Beacons (NCBs) for DNA sequence detection are not straightforward, and, thus, we carried out a series of investigations with the goal of understanding the mechanism of this template-transformation phenomenon. Experimental results showed that the six-nucleotide fragment (5'-CACCGC-3') at the 5'-end of RNuS acts as a template convertor and takes full responsibility for the template transformation from GNuS to RNuS. Moreover, we found that the appropriate proximity of the convertor to GNuS also plays a significant role in the template transformation. We then show that the insights gained here for the template-transformation mechanism allow us to construct ratiometric NCBs by simply appending the convertor and the GNuS onto a rationally designed stem-loop probe. This new type of NCB emits intense red fluorescence without the addition of a target DNA and emerges as a new, bright green emission only when hybridized to its target DNA. By measuring the distinct variation in the fluorescence intensity ratios of green and red emission, this ratiometric NCB was demonstrated to sensitively detect Hepatitis-A virus gene sequences, a proof-of-concept target in this work, with good selectivity.

Rational Design of Biomolecular Templates for Synthesizing Multifunctional Noble Metal Nanoclusters toward Personalized Theranostic Applications

Biomolecule-templated or biotemplated metal nanoclusters (NCs) are ultrasmall (<2 nm) metal (Au, Ag) particles stabilized by a certain type of biomolecular template (e.g., peptides, proteins, and DNA). Due to their unique physiochemical properties, biotemplated metal NCs have been widely used in sensing, imaging, delivery and therapy. The overwhelming applications in these individual areas imply the great promise of harnessing biotemplated metal NCs in more advanced biomedical aspects such as theranostics. Although applications of biotemplated metal NCs as theranostic agents are trending, the rational design of biomolecular templates suitable for the synthesis of multifunctional metal NCs for theranostics is comparatively underexplored. This progress report first identifies the essential attributes of biotemplated metal NCs for theranostics by reviewing the state-of-art applications in each of the four modalities of theranostics, namely sensing, imaging, delivery and therapy. To achieve high efficacy in these modalities, we elucidate the design principles underlying the use of biomolecules (proteins, peptides and nucleic acids) to control the NC size, emission color and surface chemistries for post-functionalization of therapeutic moieties. We then propose a unified strategy to engineer biomolecular templates that combine all these modalities to produce multifunctional biotemplated metal NCs that can serve as the next-generation personalized theranostic agents.

Effect of compartmentalization of donor and acceptor on the ultrafast resonance energy transfer from DAPI to silver nanoclusters

The mechanism and dynamics of excitation energy transfer (EET) from photo-excited 4',6-diamidino-2-phenylindole (DAPI) to silver nanoclusters (Ag NCs) and its subsequent modulation in the presence of cationic polymer poly(diallyldimethylammonium chloride) (PDADMAC) and Calf Thymus DNA (CT-DNA) have been demonstrated using steady-state fluorescence and femtosecond fluorescence upconversion techniques. The synthesized Ag NCs were characterized using FTIR, mass spectrometry, XPS, HRTEM, DLS, UV-Vis and PL spectroscopy. Mass spectrometric analysis reveals the formation of ultrasmall Ag4 NCs with a small amount of Ag5 NCs. UV-Vis and PL spectra reveal distinct molecular-like optoelectronic behaviour of these ultrasmall Ag NCs. The dihydrolipoic acid-capped Ag NCs strongly quench the fluorescence of DAPI with concomitant increase in its photoluminescence (PL) intensity at 675 nm. This steady-state fluorescence quenching proceeds with a significant shortening of the fluorescence lifetime of DAPI in the presence of Ag NCs, signifying the nonradiative Förster resonance energy transfer (FRET) from DAPI to Ag NCs. Various energy transfer parameters have been estimated from FRET theory. The present FRET pair shows a characteristic Förster distance of 2.45 nm and can be utilized as a reporter of short-range distances in various FRET based applications. Moreover, this nonradiative FRET is completely suppressed in the presence of both 0.2 wt% PDADMAC and CT-DNA. Our results reveal selective compartmentalization of Ag NCs and DAPI in the presence of 0.2 wt% PDADMAC and CT-DNA, respectively. This selective compartmentalization of donor and acceptor and the subsequent modification of the FRET process may find application in various sensing, photovoltaic, and light harvesting applications.

Label-free femtomolar cancer biomarker detection in human serum using graphene-coated surface plasmon resonance chips

Sensitive and selective detection of cancer biomarkers is vital for the successful diagnosis of early stage cancer and follow-up treatment. Surface Plasmon Resonance (SPR) in combination with different amplification strategies is one of the analytical approaches allowing the screening of protein biomarkers in serum. Here we describe the development of a point-of-care sensor for the detection of folic acid protein (FAP) using graphene-based SPR chips. The exceptional properties of CVD graphene were exploited to construct a highly sensitive and selective SPR chip for folate biomarker sensing in serum. The specific recognition of FAP is based on the interaction between folic acid receptors integrated through π-stacking on the graphene coated SPR chip and the FAP analyte in serum. A simple post-adsorption of human serum:bovine serum albumin (HS:BSA) mixtures onto the folic acid modified sensor resulted in a highly anti-fouling interface, while keeping the sensing capabilities for folate biomarkers. This sensor allowed femtomolar (fM) detection of FAP, a detection limit well adapted and promising for quantitative clinical analysis.

Recombinant sFRP4 bound chitosan–alginate composite nanoparticles embedded with silver nanoclusters for Wnt/β-catenin targeting in cancer theranostics

Targeting a specific pathway aberrantly upregulated in cancer cells has shown immense potential in cancer therapy.