Dissolution-Enhanced Luminescent Bioassay Based on Inorganic Lanthanide Nanoparticles

Structure-Activity Optimization of Luminescent Tb-doped LaF3 Nanoparticles

A series of Tb-doped LaF3 nanoparticles (NPs) was prepared by systematically varying the Tb doping rate from 0 to 100%. The elemental composition was confirmed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis, and the size, morphology, and crystal structure were determined in the solid state by transmission electron microscopy and X-ray diffractometry, while the size and ζ-potential of the NPs in solution were studied by dynamic light scattering, Taylor dispersion analysis, and laser Doppler electrophoresis. While the crystal structure appears to be hexagonal for a doping rate of up to 70%, an admixture of hexagonal and orthorhombic phases is observed for 80 and 90% Tb contents with a pure orthorhombic phase being obtained for TbF3. The spectroscopic properties of the NPs were studied for bare NPs and in the presence of dipicolinic acid as a surface-capping antenna ligand in solution. The coverage of the NPs by the ligand resulted in an increase in the luminescence lifetime of the emitting Tb centers, as a consequence of a better protection toward luminescence quenching from water molecules, as well as a large improvement in the brightness of the NPs. Taking into account the various parameters, a doping rate of 40% Tb was shown to be the best compromise for the development of such NPs for bioanalytical applications.

Ultrasensitive determination of surface proteins on tumor-derived small extracellular vesicles for breast cancer identification based on lanthanide-activated signal amplification strategy

Small extracellular vesicles (sEVs) carrying multiple tumor-associated proteins inherited from parental cells play crucial roles in noninvasive breast cancer (BC) diagnosis. However, it is challenging to assess the subtle variations of surface proteins on sEV membranes due to the highly heterogeneous BC. Therefore, a simple and ultrasensitive assay based on lanthanide (Ln3+)-activated luminescence signal amplification was developed to detect multiple surface proteins on BC-derived sEVs. Multiple protein biomarkers on sEVs can be well identified with high sensitivity and specificity through dissolution-amplified luminescence of the NaEuF4 nanoparticle-based nanoprobe. We employ linear discriminant analysis to successfully discriminate triple negative BC cell (MDA-MB-231 cell) derived sEVs from other breast cell lines (MCF-7, SK-BR-3, BT474 and MCF-10A cell). Furthermore, the strategy enables high accuracy for districting the progression stages of BC patients and healthy donors. The simple and sensitive signal amplification strategy exhibits great potential for early clinic diagnosis by precise protein profiling of sEVs.

Ultrasensitive Urinary Diagnosis of Organ Injuries Using Time-Resolved Luminescent Lanthanide Nano-bioprobes

Urinary sensing of synthetic biomarkers that are released into urine after specific activation in an in vivo disease environment is an emerging diagnosis strategy to overcome the insensitivity of a previous biomarker assay. However, it remains a great challenge to achieve sensitive and a specific urinary photoluminescence (PL) diagnosis. Herein, we report a novel urinary time-resolved PL (TRPL) diagnosis strategy by exploiting europium complexes of diethylenetriaminepentaacetic acid (Eu-DTPA) as synthetic biomarkers and designing the activatable nanoprobes. Notably, TRPL of Eu-DTPA in the enhancer can eliminate the urinary background PL for ultrasensitive detection. We achieved sensitive urinary TRPL diagnosis of mice kidney and liver injuries by using simple Eu-DTPA and Eu-DTPA-integrated nanoprobes, respectively, which cannot be realized by traditional blood assays. This work demonstrates the exploration of lanthanide nanoprobes for in vivo disease-activated urinary TRPL diagnosis for the first time, which might advance the noninvasive diagnosis of diverse diseases via tailorable nanoprobe designs.

Dissolution-Enhanced Luminescence Enhanced Digital Microfluidics Immunoassay for Sensitive and Automated Detection of H5N1

Protein bioassay is a critical tool for the screening and detection of protein biomarkers in disease diagnostics and biological applications. However, the detection sensitivity and system automation of current immunoassays do not meet the emerging demands of clinical applications. Here, we developed a dissolution-enhanced luminescence-enhanced digital microfluidics immunoassay (DEL-DMF), which significantly improves the sensitivity and automation of the protein bioassay. In DEL-DMF, the sample and reagent droplets are controlled to complete the processes of sample transport, immunoreaction, and buffer washing, which not only minimizes sample consumption to 2 μL and enhances the binding efficiency of immunoreaction but also streamlines all the procedures and simplifies the process of immunoassay. Moreover, dissolution-enhanced luminescence using NaEuF₄ NPs as nanoprobes boosts the fluorescence and increases the sensitivity of the bioassay. We demonstrate the enhanced analytical performance of our DEL-DMF immunoassay to detect H5N1 hemagglutinin in human serum and saliva. A limit of detection of 1.16 pM was achieved in less than 0.5 h with only 2 μL sample consumption. Overall, our DEL-DMF immunoassay combines the merits of the microfluidics platform and dissolution-enhanced luminescence, thus affording superior detection sensitivity and system automation for protein biomarkers. This novel immunoassay microsystem holds great potential in clinical and biological applications.

High-Contrast Luminescent Immunohistochemistry Using PEGylated Lanthanide Complexes

Immunohistochemistry (IHC) using fluorescent probes provides high resolution with multiplexing capability, but the imaging contrast is limited by the brightness of the fluorescent probe and the intrinsic autofluorescence background from tissues. Herein, we improved the contrast by high-density labeling of long-lifetime lanthanide complexes and time-gated imaging. As the large (∼280 nm) Stokes shift of lanthanide complexes effectively prevents the issue of concentration quenching, we succeeded in conjugating seven europium complexes to an eight-arm hydrophilic poly(ethylene glycol) (PEG) linker for signal amplification with improved water solubility to the level of up to 10 mg/mL. Moreover, we demonstrated that both human epidermal growth factor receptor 2 (HER2) in a formalin-fixed paraffin-embedded (FFPE) tissue section and cytokeratin 18 (CK18) in a frozen section can be resolved with the enhanced contrast by 2-fold and 3-fold, respectively. Furthermore, we show that the PEGylation of multiple lanthanide complexes is compatible with tyramide signal amplification (TSA). This work suggests new opportunities for sensitive imaging of low-abundance biomarkers in a tissue matrix.

Integration and Quantitative Visualization of 3,3',5,5'-Tetramethylbenzidine-Probed Enzyme-Linked Immunosorbent Assay-like Signals in a Photothermal Bar-Chart Microfluidic Chip for Multiplexed Immunosensing

The photothermal effect shows significant promise for various biomedical applications but is rarely exploited for microfluidic lab-on-a-chip bioassays. Herein, a photothermal bar-chart microfluidic immunosensing chip, with the integration of the conventional 3,3',5,5'-tetramethylbenzidine (TMB)-probed enzyme-linked immunosorbent assay (ELISA)-like system, was developed based on exploiting the photothermal pumping technique for visual bar-chart microfluidic immunosensing. Both the sandwich ELISA-like system and the photothermal pumping protocol were integrated into a single photothermal bar-chart chip. On-chip immunocaptured iron oxide nanoparticles catalyzed the oxidation of the chromogenic substrate, TMB, to produce a sensitive photothermal and chromogenic dual-functional probe, oxidized TMB. As the result of heat generation and the subsequent production of elevating vapor pressure in the sealed microfluidic environment, the on-chip near-infrared laser-driven photothermal effect of the probe served as a dose-dependent pumping force to drive the multiplexed quantitative display of the immunosensing signals as visual dye bar charts. Prostate-specific antigen as a model analyte was tested at a limit of detection of 1.9 ng·mL^-1, lower than the clinical diagnostic threshold of prostate cancer. This work presents a new perspective for microfluidic integration and multiplexed quantitative bar-chart visualization of the conventional TMB-probed ELISA signals possibly by means of an affordable handheld laser pointer in a lab-on-a-chip format.

Functional Micro-/Nanomaterials for Multiplexed Biodetection

When analyzing biological phenomena and processes, multiplexed biodetection has many advantages over single-factor biodetection and is highly relevant to both human health issues and advancements in the life sciences. However, many key problems with current multiplexed biodetection strategies remain unresolved. Herein, the main issues are analyzed and summarized: 1) generating sufficient signal to label targets, 2) improving the signal-to-noise ratio to ensure total detection sensitivity, and 3) simplifying the detection process to reduce the time and labor costs of multiple target detection. Then, available solutions made possible by designing and controlling the properties of micro- and nanomaterials are introduced. The aim is to emphasize the role that micro-/nanomaterials can play in the improvement of multiplexed biodetection strategies. Through analyzing existing problems, introducing state-of-the-art developments regarding relevant materials, and discussing future directions of the field, it is hopeful to help promote necessary developments in multiplexed biodetection and associated scientific research.

Detector-Free Photothermal Bar-Chart Microfluidic Chips (PT-Chips) for Visual Quantitative Detection of Biomarkers

The volumetric bar-chart microfluidic chips (V-Chips) driven by chemical reaction-generated gas provide a promising platform for point-of-care (POC) visual biomarker quantitation. However, multiple limitations are encountered in conventional V-Chips, such as costly and complex chip fabrication, complicated chip assembly, and imprecise controllability of gas production. Herein, we introduced nanomaterial-mediated photothermal effects to V-Chips, and for the first time developed a new type of V-Chip, photothermal bar-chart microfluidic chip (PT-Chip), for visual quantitative detection of biochemicals without any bulky and costly analytical instruments. Immunosensing signals were converted to visual readout signals via photothermal effects, the on-chip bar-chart movements, enabling quantitative biomarker detection on a low-cost polymer hybrid PT-Chip with on-chip scale rulers. Four different human serum samples containing a prostate-specific antigen (PSA) as a model analyte were detected simultaneously using the PT-Chip, with a limit of detection of 2.1 ng/mL, meeting clinical diagnostic requirements. Although no conventional signal detectors were used, it achieved comparable detection sensitivity to absorbance measurements with a microplate reader. The PT-Chip was further validated by testing human whole blood without the color interference problem, demonstrating the good analytical performance of our method even in complex matrices and thus the potential to fill the gap in current clinical diagnostics that is incapable of testing whole blood. This new PT-Chip driven by nanomaterial-mediated photothermal effects opens a new horizon of microfluidic platforms for instrument-free diagnostics at the point-of-care.

Lanthanide-Based Optical Probes of Biological Systems

The unique photophysical properties of lanthanides, such as europium, terbium, and ytterbium, make them versatile molecular probes of biological systems. In particular, their long-lived photoluminescence, narrow bandwidth emissions, and large Stokes shifts enable experiments that are infeasible with organic fluorophores and fluorescent proteins. The ability of these metal ions to undergo luminescence resonance energy transfer, and photon upconversion further expands the capabilities of lanthanide probes. In this review, we describe recent advances in the design of lanthanide luminophores and their application in biological research. We also summarize the latest detection systems that have been developed to fully exploit the optical properties of lanthanide luminophores. We conclude with a discussion of remaining challenges and new frontiers in lanthanide technologies. The unprecedented levels of sensitivity and multiplexing afforded by rare-earth elements illustrate how chemistry can enable new approaches in biology.

Structure Engineering of a Lanthanide-Based Metal-Organic Framework for the Regulation of Dynamic Ranges and Sensitivities for Pheochromocytoma Diagnosis

Exploring innovative technologies to precisely quantify biomolecules is crucial but remains a great challenge for disease diagnosis. Unfortunately, the humoral concentrations of most biotargets generally vary within rather limited scopes between normal and pathological states, while most literature-reported biosensors can detect large spans of targets concentrations, but are less sensitive to small concentration changes, which consequently make them mostly unsatisfactory or even unreliable in distinguishing positives from negatives. Herein, a novel strategy of precisely quantifying the small concentration changes of a certain biotarget by editing the dynamic ranges and sensitivities of a lanthanide-based metal-organic framework (Eu-ZnMOF) biosensor is reported. By elaborately tailoring the biosensor's structure and surface areas, the tunable Eu-ZnMOF is developed with remarkably enhanced response slope within the "optimized useful detection window," enabling it to serve as a powerful signal amplifier (87.2-fold increase) for discriminating the small concentration variation of urinary vanillylmandelic acid (an early pathological signature of pheochromocytoma) within only three times between healthy and diseased subjects. This study provides a facile approach to edit the biosensors' performances through structure engineering, and exhibits promising perspectives for future clinical application in the non-invasive and accurate diagnosis of severe diseases.

Lanthanide Metal-Organic Framework Nanoprobes for the In Vitro Detection of Cardiac Disease Markers

Acute myocardial infarction (AMI) is one of the leading causes of death around the world. An early and accurate diagnosis of AMI is critical to reduce the mortality rate. As an important cardiac biomarker, creatine kinase (CK) has been used in the clinical diagnosis of AMI. However, it still remains a great challenge to realize highly sensitive and selective CK detection in blood specimens. Herein, we have developed an ultrasensitive platform for the detection of CK activity based on time-resolved (TR) luminescent lanthanide metal-organic framework nanoprobes (Eu-QPTCA). Benefiting from the intense emission of lanthanide ions sensitized by the organic ligands and the eliminated short-lived autofluorescence by the TR technique, these nanoprobes enabled the homogeneous detection of CK activity with a limit of detection down to 1.0 U/L, which is about 1 order of magnitude improvement relative to that of the traditional methods. In addition, the Eu-QPTCA nanoprobes showed superior selectivity and reliability toward the practical detection of CK activity in human serum, indicating the great significance of our method in the early diagnosis of AMI. We envision that the proposed bioassay strategy can be extended to the detection of other phosphorylation enzymes, paving a way for promising applications in clinical diagnostics.

Intense near-infrared-II luminescence from NaCeF4:Er/Yb nanoprobes for in vitro bioassay and in vivo bioimaging

Near-infrared (NIR) II luminescence between 1000 and 1700 nm has attracted reviving interest for biosensing due to its unique advantages such as deep-tissue penetration and high spatial resolution. Traditional NIR-II probes such as organic fluorophores usually suffer from poor photostability and potential long-term toxicity. Herein, we report the controlled synthesis of monodisperse NaCeF₄:Er/Yb nanocrystals (NCs) that exhibit intense NIR-II emission upon excitation at 980 nm. Ce³⁺ in the host lattice was found to enhance the luminescence of Er³⁺ at 1530 nm with a maximum NIR-II quantum yield of 32.8%, which is the highest among Er³⁺-activated nanoprobes. Particularly, by utilizing the intense NIR-II emission of NaCeF₄:Er/Yb NCs, we demonstrated their application as sensitive homogeneous bioprobes to detect uric acid with the limit of detection down to 25.6 nM. Furthermore, the probe was detectable in tissues at depths of up to 10 mm, which enabled in vivo imaging of mouse organs and hindlimbs with high resolution, thus revealing the great potential of these NaCeF₄:Er/Yb nanoprobes in deep-tissue diagnosis.

Multiplex Analysis on a Single Porous Hydrogel Bead with Encoded SERS Nanotags

Bead-based assays have drawn more and more attention in biomedical fields. Herein, we proposed a novel approach to achieve multiplex analysis on a single porous hydrogel bead (PHB) with Raman dyes (RDs) encoded core-shell surface-enhanced Raman scattering (SERS) nanotags. Because of the amplified signal of RDs by core-shell metal structure of the nanotag and the high surface area to volume ratio (SVR) of the PHB, the sensitivity and linear dynamic range (LDR) of the as-proposed multiplex analysis method are significantly improved. We anticipate this approach to be widely used in the multiplex assays and biosensors.

Lanthanide-Doped Nanoparticles for Diagnostic Sensing

Lanthanide-doped nanoparticles exhibit unique optical properties, such as a long luminescence lifetime (up to several milliseconds), sharp emission peaks, and upconversion luminescence over the range of wavelengths from near-infrared to visible. Exploiting these optical properties, lanthanide-doped nanoparticles have been widely utilized for cellular and small animal imaging with the absence of background autofluorescence. In addition, these nanoparticles have advantages of high signal-to-noise ratio for highly sensitive and selective diagnostic detection. In this review, we summarize and discuss recent progress in the development of highly sensitive diagnostic methods using lanthanide-doped nanoparticles. Combined with a smartphone, portable luminescence detecting platforms could be widely applied in point-of-care tests.

In vitro upconverting/downshifting luminescent detection of tumor markers based on Eu3+-activated core-shell-shell lanthanide nanoprobes

Trivalent europium (Eu³⁺) doped inorganic nanoparticles (NPs), emerging as a new class of red luminescent nanoprobes, have shown great promise in bioapplications as diverse as luminescent bioassays and disease theranostics owing to their superior optical properties such as long-lived downshifting luminescence (DSL) and upconverting luminescence (UCL). However, the exploration of Eu³⁺-doped NPs as red luminescent bioprobes particularly combined with DSL and UCL of Eu³⁺ hitherto remains untouched. Herein, we report a rational core-shell-shell (CSS) design strategy to construct Eu³⁺-activated NaGdF₄:Yb/Tm@NaGdF₄:Eu@NaEuF₄ CSS NPs functionalized with efficient UCL and dissolution-enhanced DSL of Eu³⁺ for in vitro tumor marker detection and tumor-targeted imaging. By utilizing the CSS NPs as red luminescent nanoprobes, we demonstrate the successful UCL and DSL bioassays of a typical hepatic carcinoma biomarker, alpha-fetoprotein (AFP), in human serum samples. The UCL bioassay shows a limit of detection (LOD) of AFP down to 20 pg mL^-1 (290 fM), which is the lowest among luminescent bioassays of AFP ever reported, and a 30-fold improvement relative to that of the commercial dissociation-enhanced lanthanide fluoroimmunoassay kit. Meanwhile the DSL bioassay, by employing the identical CSS NPs, can serve as a self-referential validation for the reliability and accuracy of the UCL bioassay for AFP detection. Furthermore, these CSS NPs can also function well in tumor-targeted UCL bioimaging, thereby revealing the great promise of the designed CSS NPs as red luminescent bioprobes in ultrasensitive in vitro detection of tumor markers in clinical diagnosis.

Ultrasensitive Detection of Ebola Virus Oligonucleotide Based on Upconversion Nanoprobe/Nanoporous Membrane System

Ebola outbreaks are currently of great concern, and therefore, development of effective diagnosis methods is urgently needed. The key for lethal virus detection is high sensitivity, since early-stage detection of virus may increase the probability of survival. Here, we propose a luminescence scheme of assay consisting of BaGdF5:Yb/Er upconversion nanoparticles (UCNPs) conjugated with oligonucleotide probe and gold nanoparticles (AuNPs) linked with target Ebola virus oligonucleotide. As a proof of concept, a homogeneous assay was fabricated and tested, yielding a detection limit at picomolar level. The luminescence resonance energy transfer is ascribed to the spectral overlapping of upconversion luminescence and the absorption characteristics of AuNPs. Moreover, we anchored the UCNPs and AuNPs on a nanoporous alumina (NAAO) membrane to form a heterogeneous assay. Importantly, the detection limit was greatly improved, exhibiting a remarkable value at the femtomolar level. The enhancement is attributed to the increased light-matter interaction throughout the nanopore walls of the NAAO membrane. The specificity test suggested that the nanoprobes were specific to Ebola virus oligonucleotides. The strategy combining UCNPs, AuNPs, and NAAO membrane provides new insight into low-cost, rapid, and ultrasensitive detection of different diseases. Furthermore, we explored the feasibility of clinical application by using inactivated Ebola virus samples. The detection results showed great potential of our heterogeneous design for practical application.

A Supramolecular Sensor Array Using Lanthanide-Doped Nanoparticles for Sensitive Detection of Glyphosate and Proteins

Lanthanide (Ln(3+))-doped nanoparticles (NPs) are an intensive area of research in chemical and materials sciences. Herein a sensor array of Ln(3+)-doped NPs was developed for the first time toward sensitive molecular sensing based on a novel strategy of the hybridized time-resolved Förster resonance energy transfer (TR-FRET) with the indicator displacement assay (IDA) concept (TR-FRET-IDA). The sensor platform was generated in situ by binding a series of negatively charged indicators on the surface of ligand-free LiYF4:Ce/Tb NPs. The TR-FRET between NPs and dyes resulted in indicator emission and was employed as a means of removing undesired short-lived background luminescence from the indicator effectively. Displacement of indicators from the NP/indicator ensembles by glyphosate, a common herbicide, led to turn-off of the indicator emission. The sensor array was able to successfully discriminate 11 biologically relevant anions with high accuracy and sensitivity in pure aqueous buffer both qualitatively and quantitatively. Furthermore, the differentiation of six model proteins in the nM range was achieved with 100% accuracy for the classification, thereby demonstrating the versatility of this simple sensor platform. The study of the mechanism of binding and signal modulation further verified TR-FRET-IDA as a reliable sensing paradigm.

Sub-5 nm lanthanide-doped lutetium oxyfluoride nanoprobes for ultrasensitive detection of prostate specific antigen

It remains challenging to develop ultrasmall (<5 nm) but highly luminescent bioprobes with a large linear detection range for the early diagnosis and monitoring of prostate cancer (PCa). Benefiting from the high molar density of lanthanide ions in an oxyfluoride matrix and the superior dissolution capability of Lu₆O₅F₈ nanoparticles in the enhancer solution, we demonstrated the successful use of novel sub-5 nm Lu₆O₅F₈:Eu³⁺ nanoprobes for the detection of prostate specific antigen (PSA) in clinical serum samples. The limit of detection for PSA is as low as 0.52 pg mL^-1, which is almost a 200-fold improvement relative to that of a commercial dissociation-enhanced lanthanide fluoroimmunoassay (DELFIA) kit. The PSA levels detected in 23 patient serum samples were consistent with those measured independently by the DELFIA kit, showing the assay's reliability with a correlation coefficient of 97%. A linear range of 4 orders of magnitude ranging from 8.5 × 10^-4 to 5.6 ng mL^-1 for the assay of PSA was achieved, which is highly promising for the early diagnosis of PCa and monitoring of PCa relapse of patients after radical prostatectomy.

One-Step Protein Conjugation to Upconversion Nanoparticles

The emerging upconversion nanoparticles offer a fascinating library of ultrasensitive luminescent probes for a range of biotechnology applications from biomarker discovery to single molecule tracking, early disease diagnosis, deep tissue imaging, and drug delivery and therapies. The effective bioconjugation of inorganic nanoparticles to the molecule-specific proteins, free of agglomeration, nonspecific binding, or biomolecule deactivation, is crucial for molecular recognition of target molecules or cells. The current available protocols require multiple steps which can lead to low probe stability, specificity, and reproducibility. Here we report a simple and rapid protein bioconjugation method based on a one-step ligand exchange using the DNAs as the linker. Our method benefits from the robust DNA-protein conjugates as well as from multiple ions binding capability. Protein can be preconjugated via an amino group at the 3' end of a synthetic DNA molecule, so that the 5' end phosphoric acid group and multiple phosphate oxygen atoms in the phosphodiester bonds are exposed to replace the oleic acid ligands on the surface of upconversion nanoparticles due to their stronger chelating capability to lanthanides. We demonstrated that our method can efficiently pull out the upconversion nanoparticles from organic solvent into an aqueous phase. The upconversion nanoparticles then become hydrophilic, stable, and specific biomolecules recognition. This allows us to successfully functionalize the upconversion nanoparticles with horseradish peroxidise (HRP) for catalytic colorimetric assay and for streptavidin (SA)-biotin immunoassays.