A sandwich FRET biosensor for lysozyme detection based on peptide-functionalized gold nanoparticles and FAM-labeled aptamer

Lysozyme (LYZ) plays a crucial role in the body's immune defense system. Monitoring LYZ levels can provide valuable insights into the diagnosis and severity assessment of various diseases. Traditionally, antibody-based sandwich assays are employed for LYZ detection, but they are often time-consuming and operationally complicated. In this research, a novel sandwich FRET biosensor was developed, which enables rapid detection of LYZ based on peptide-functionalized gold nanoparticles (pAuNPs) and FAM-labeled aptamer (Apt-FAM). Initially, a mixture of Apt-FAM and pAuNPs resulted in partial quenching of the Apt-FAM fluorescence emission through an inner filter effect (IFE), with negligible energy transfer because of the electrostatic repulsion between the negatively charged pAuNPs and Apt-FAM. The introduction of LYZ into the mixture drove the specific binding of Apt-FAM and pAuNPs to LYZ, facilitating the formation of a pAuNPs-LYZ-aptamer sandwich structure. The formation of this complex drew the pAuNPs and Apt-FAM into close enough proximity to enable FRET to occur, which in turn effectively quenched the fluorescence emission of FAM. The decrease in FAM fluorescence intensity was correlated with the increasing concentration of LYZ. Thus, a sandwich FRET biosensor was successfully developed for LYZ detection with a linear detection range of 0-1.75 μM and a detection limit of 85 nM. Additionally, the biosensor allowed visual detection of LYZ in a 96-well microplate, with a rapid response time of just 15 s. This study introduces a innovative sandwich FRET biosensor that combines aptamer and peptide recognition elements, offering a fast and antibody-free method for protein detection.

Unlabelled LRET biosensor based on double-stranded DNA for the detection of anthraquinone anticancer drugs

Based on upconversion nanoparticles (UCNPs) as energy donor and herring sperm DNA (hsDNA) as molecular recognition element, an unlabelled upconversion luminescence (UCL) affinity biosensor was constructed for the detection of anthraquinone (AQ) anticancer drugs in biological fluids. AQ anticancer drugs can insert into the double helix structure of hsDNA on the surface of UCNPs, thereby shortening the distance from UCNPs. Therefore, the luminescence resonance energy transfer (LRET) phenomenon is effectively triggered between UCNPs and AQ anticancer drugs. Hence, AQ anticancer drugs can be quantitatively detected according to the UCL quenching rate. The biosensor showed good sensitivity and stability for the detection of daunorubicin (DNR) and doxorubicin (ADM). For the detection of DNR, the linear range is 1-100 μg·mL-1 with a limit of detection (LOD) of 0.60 μg·mL-1, and for ADM, the linear range is 0.5-100 μg·mL-1 with a LOD of 0.38 μg·mL-1. The proposed biosensor provides a convenient method for monitoring AQ anticancer drugs in clinical biological fluids in the future.

Upconversion luminescence-based aptasensor for the detection of thyroid-stimulating hormone in serum

Thyroid-stimulating hormone (TSH) plays a crucial physiological and pathological role in humans, and a timely and sensitive detection of TSH is critical for early diagnosis and prevention of thyroid-related diseases. Herein, we developed a simple wash-free biological aptasensor based on luminescence resonance energy transfer (LRET) between NaYF₄:Yb,Er upconversion nanoparticles (UCNPs) and tetramethylrhodamine (TAMRA) for the detection of TSH with high sensitivity. In this LRET system, UCNPs as donors and TAMRA as receptors were modified with nucleic acid aptamers Apt-1 and Apt-2, respectively. When TSH was present, the two aptamer strands both specifically recognized TSH to form a hairpin-like structure, thereby shortening the space between UCNPs and TAMRA. The LRET occurred under radiation of 980-nm light. By detecting the change of upconversion luminescence (UCL) intensity (I_545nm), the activity of TSH was quantified. The resulting detection dynamic range and the limit of detection were 0.1-5.0 mIU·L^-1 and 0.065 mIU·L^-1, respectively. The aptasensor using UCNPs as LRET donors was capable of effectively eliminating the background interference of a complicated biological environment, and showed good specificity because of the excellent recognition function of aptamers. Due to high sensitivity, easiness of fabrication, operational convenience, and selectivity, the UCL-based aptasensor is a promising candidate for clinical TSH determination. Based on nucleic acid aptamer and the mechanism of luminescence resonance energy transfer (LRET) between upconversion nanoparticles (UCNPs) donor and tetramethylrhodamine (TAMRA) receptor, an aptasensor was constructed for the quantitative analysis of TSH activity in serum by testing the change of I_545nm.

Exploring the use of upconversion nanoparticles in chemical and biological sensors: from surface modifications to point-of-care devices

Upconversion nanoparticles (UCNPs) have emerged as promising luminescent nanomaterials due to their unique features that allow the overcoming of several problems associated with conventional fluorescent probes. Although UCNPs have been used in a broad range of applications, it is probably in the field of sensing where they best evidence their potential. UCNP-based sensors have been designed with high sensitivity and selectivity, for detection and quantification of multiple analytes ranging from metal ions to biomolecules. In this review, we deeply explore the use of UCNPs in sensing systems emphasizing the most relevant and recent studies on the topic and explaining how these platforms are constructed. Before diving into UCNP-based sensing platforms it is important to understand the unique characteristics of these nanoparticles, why they are attracting so much attention, and the most significant interactions occurring between UCNPs and additional probes. These points are covered over the first two sections of the article and then we explore the types of fluorescent responses, the possible analytes, and the UCNPs' integration with various material types such as gold nanostructures, quantum dots and dyes. All the topics are supported by analysis of recently reported sensors, focusing on how they are built, the materials' interactions, the involved synthesis and functionalization mechanisms, and the conjugation strategies. Finally, we explore the use of UCNPs in paper-based sensors and how these platforms are paving the way for the development of new point-of-care devices.

Sensors/nanosensors based on upconversion materials for the determination of pharmaceuticals and biomolecules: An overview

Upconversion materials have been the focus of a large body of research in analytical and clinical fields in the last two decades owing to their ability to convert light between various spectral regions and their particular photophysical features. They emit efficient and sharp ultraviolet (UV) or visible luminescence after excitation with near-infrared (NIR) light. These features overcome some of the disadvantages reported for conventional fluorescent materials and provide opportunities for high sensitivity chemo-and bio-sensing. Here, we review studies that used upconversion materials as sensors for the determination of pharmaceuticals and biomolecules in the last two decades. The articles included in this review were retrieved from the SCOPUS database using the search phrases: "upconversion nanoparticles for determination of pharmaceutical compounds", and "upconversion nanoparticles for determination of biomolecules". Details of each developed upconversion nanoparticles based sensor along with their relevant analytical parameters are reported and carefully explained.

Dual-Acceptor-Based Upconversion Luminescence Nanosensor with Enhanced Quenching Efficiency for in Situ Imaging and Quantification of MicroRNA in Living Cells

Upconversion nanoparticles (UCNPs) have become competitive materials for bioanalysis, bioimaging, and early diagnosis of diseases, especially cancers. However, traditional upconversion luminescence (UCL) nanosensors are often challenged with complicated covalent modification and relatively poor stability. As efficient energy acceptors in the luminescence resonance energy-transfer (LRET) process, organic dyes exhibit unique advantages such as easy modification and stable property. Herein, a simple and universal bioplatform is constructed for in situ imaging and quantitation of intracellular microRNA-21 (miR-21) using dual-acceptor-based upconversion nanoprobes with enhanced quenching efficiency. In this assay, UCNPs with core-shell structures are synthesized, in which the emitting ions are confined in the shell to take the energy donors and acceptors in close proximity. The complementary DNA (cDNA) that can specifically recognize target miR-21 is labeled with organic dyes TAMRA and black hole quencher as dual acceptors and easily assembled on UCNPs via electrostatic adsorption. Compared with only one acceptor for LRET, two dyes quench more luminescence of UCNPs (>60%), which thus reduce the background and improve the sensitivity. With the enhanced quenching efficiency and simple assembly process, the proposed system is readily applied to in situ imaging of miR-21 in different cancer cells, which further achieves quantification of miR-21 in MCF-7 cells. Therefore, our proposed dual-acceptor-based upconversion nanoplatform opens up new opportunities for sensitive analysis of miRNA and provides potential applications in biomedical and clinical research.

Sensitive and specific detection of proteins based on target-responsive DNA polymerase activity

We herein describe a novel method for the detection of target protein based on the target-responsive DNA polymerase activity. In the sensor, two different types of DNA aptamers with the respective functions: one binds to the target protein and the other binds to DNA polymerase, are rationally engineered and combined to form the detection probe that regulates DNA polymerase activity in response to the target protein. In the presence of target protein, the detection probe becomes destabilized and stops the inhibition of DNA polymerase activity. Consequently, the active DNA polymerase initiates the primer extension reaction on the target-specific DNA aptamer, which recycles the target protein to promote another activation cycle of DNA polymerase. In addition, DNA polymerase also catalyzes the primer extension reaction on the primer/template complex in conjugation with TaqMan probe, leading to the significantly enhanced fluorescence intensities. With this novel strategy, we detected a model target protein, lysozyme with a limit of detection as low as 0.80 nM. In addition, the practical applicability of this system was successfully demonstrated by determining lysozyme in human serum.

Stable gadolinium based nanoscale lyophilized injection for enhanced MR angiography with efficient renal clearance

There is a great demand to develop high-relaxivity nanoscale contrast agents for magnetic resonance (MR) angiography with high resolution. However, there should be more focus on stability, ion leakage and excretion pathway of the intravenously injected nanoparticles, which are closely related to their clinic potentials. Herein, uniform ultrasmall-sized NaGdF₄ nanocrystal (sub-10 nm) was synthesized using a facile high temperature organic solution method, and the nanocrystals were modified by a ligand-exchange approach using PEG-PAA di-block copolymer. The PEG-PAA modified NaGdF₄ nanocrystal (denoted as ppNaGdF₄ nanocrystal) exhibited a high r₁ relaxivity which was twice of commercially used gadopentetate dimeglumine (Gd-DTPA) injection. MR angiography on rabbit using ppNaGdF₄ nanocrystal at a low dose showed enhanced vascular details and long circulation time. Lyophilized powder of ppNaGdF₄ nanocrystals have been successfully prepared without aggregation or reduction of MR performance, indicating the stability and an effective way to store this nanoscale contrast agent. No haemolysis was induced by ppNaGdF₄ nanocrystal, and an extremely low leakage of gadolinium ions was confirmed. Furthermore, efficient renal excretion was one of the clearance pathways of ppNaGdF₄ nanocrystal according to both the time dependent distribution data in blood and tissues and MR images. The in vivo toxicity evaluation further validated the great potential as a clinical agent for blood pool imaging.

Sequential growth of CaF2:Yb,Er@CaF2:Gd nanoparticles for efficient magnetic resonance angiography and tumor diagnosis

It is a significant challenge to develop nanoscale magnetic resonance imaging (MRI) contrast agents with high performance of relaxation.

A Fibrous Localized Drug Delivery Platform with NIR-Triggered and Optically Monitored Drug Release

Implantable localized drug delivery systems (LDDSs) with intelligent functionalities have emerged as a powerful chemotherapeutic platform in curing cancer. Developing LDDSs with rationally controlled drug release and real-time monitoring functionalities holds promise for personalized therapeutic protocols but suffers daunting challenges. To overcome such challenges, a series of porous Yb(3+)/Er(3+) codoped CaTiO3 (CTO:Yb,Er) nanofibers, with specifically designed surface functionalization, were synthesized for doxorubicin (DOX) delivery. The content of DOX released could be optically monitored by increase in the intensity ratio of green to red emission (I550/I660) of upconversion photoluminescent nanofibers under 980 nm near-infrared (NIR) excitation owing to the fluorescence resonance energy transfer (FRET) effect between DOX molecules and the nanofibers. More importantly, the 808 nm NIR irradiation enabled markedly accelerated DOX release, confirming representative NIR-triggered drug release properties. In consequence, such CTO:Yb,Er nanofibers presented significantly enhanced in vitro anticancer efficacy under NIR irradiation. This study has thus inspired another promising fibrous LDDS platform with NIR-triggered and optics-monitored DOX releasing for personalized tumor chemotherapy.

An upconversion fluorescent resonant energy transfer biosensor for hepatitis B virus (HBV) DNA hybridization detection

A novel fluorescent resonant energy transfer (FRET) biosensor was fabricated for the detection of hepatitis B virus (HBV) DNA using poly(ethylenimine) (PEI) modified upconversion nanoparticles (NH2-UCNPs) as energy donor and gold nanoparticles (Au NPs) as acceptor. The PEI modified upconversion nanoparticles were prepared directly with a simple one-pot hydrothermal method, which provides high quality amino-group functionalized UCNPs with uniform morphology and strong upconversion luminescence. Two single-stranded DNA strands, which were partially complementary to each other, were then conjugated with NH2-UCNPs and Au NPs. When DNA conjugated NH2-UCNPs and Au NPs are mixed together, the hybridization between complementary DNA sequences on UCNPs and Au NPs will lead to the quenching of the upconversion luminescence due to the FRET process. Meanwhile, upon the addition of target DNA, Au NPs will leave the surface of the UCNPs and the upconversion luminescence can be restored because of the formation of the more stable double-stranded DNA on the UCNPs. The sensor we fabricated here for target DNA detection shows good sensitivity and high selectivity, which has the potential for clinical applications in the analysis of HBV and other DNA sequences.

Fluorescent nanoprobes for sensing and imaging of metal ions: recent advances and future perspectives

Recent advances in nanoscale science and technology have generated nanomaterials with unique optical properties. Over the past decade, numerous fluorescent nanoprobes have been developed for highly sensitive and selective sensing and imaging of metal ions, both in vitro and in vivo. In this review, we provide an overview of the recent development of the design and optical properties of the different classes of fluorescent nanoprobes based on noble metal nanomaterials, upconversion nanoparticles, semiconductor quantum dots, and carbon-based nanomaterials. We further detail their application in the detection and quantification of metal ions for environmental monitoring, food safety, medical diagnostics, as well as their use in biomedical imaging in living cells and animals.

Single-layer MoS2 nanosheet grafted upconversion nanoparticles for near-infrared fluorescence imaging-guided deep tissue cancer phototherapy

A multifunctional nanostructure is prepared by covalently grafting upconversion nanoparticles (UCNPs) with chitosan functionalized MoS2 (MoS2-CS) and folic acid (FA) and then loading phthalocyanine (ZnPc) on the surface of MoS2, which integrates photodynamic therapy (PDT) with photothermal therapy (PTT) and upconversion luminescence imaging into one system for enhanced antitumor efficiency.

Surface functionalization of up-converting NaYF4 nanocrystals with chiral molecules

The surface of up-converting NaYF₄:2%Er,20%Yb NPs have been successfully functionalized with chiral molecules, with simultaneously preserved colloidal stability and intense up-conversion emission.

Novel nonplanar and rigid fluorophores with intensive emission in water and the application in two-photon imaging of live cells

Novel nonplanar and rigid fluorophores have been synthesized, by fusing a twisted heterocycle onto a naphthalimide skeleton, and exhibited excellent higher quantum yield value (Φ = 0.60–0.66) and molar extinction coefficient in water.

pH-Triggered SrTiO3:Er Nanofibers with Optically Monitored and Controlled Drug Delivery Functionality

The design of multifunctional localized drug delivery systems (LDDSs) has been endeavored in the past decades worldwide. The matrix material of LDDSs is known as a crucial factor for the success of its transformation from the laboratory to clinical practices. Herein, a biocompatible ceramic, strontium titanate (SrTiO3, STO), was utilized as the matrix. A variety of fine Er doped SrTiO3 (STO:Er) nanofibers were fabricated via electrospinning. After the surface functionalization with amino groups, the drug loading capacity of STO:Er nanofibers is dramatically increased. The nanofibers present a rather sustained drug releasing behavior in the media with pH of 7.4, and the release kinetics is significantly accelerated with the decreased pH value from 7.4 to 4.7. Furthermore, the intensity of the spectrum emitted from the STO:Er nanofibers corresponds well with the drug releasing progress under the excitation of near-infrared spectrum (∼980 nm). Fast drug release behavior (in an acid environment) induces a rapid intensity enhancing effect of photoluminescence emission and vice versa. The main mechanism is attributed to the quenching effect induced by the C-Hx groups of IBU molecules with vibration frequencies from 2850 to 3000 cm(-1). Such new STO:Er nanofibers with pH-triggered and optically monitored drug delivery functionalities have therefore been considered as another new localized drug delivery platform for modern tumor diagnosis and therapy.