Tuning the dual emission of photon-upconverting nanoparticles for ratiometric multiplexed encoding

Nanoscale potassium sensing based on valinomycin-anchored fluorescent gold nanoclusters

Gold nanoclusters are a smart platform for sensing potassium ions (K+). They have been synthesized using bovine serum albumin (BSA) and valinomycin (Val) to protect and cap the nanoclusters. The nanoclusters (Val-AuNCs) produced have a red emission at 616 nm under excitation with 470 nm. In the presence of K+, the valinomycin polar groups switch to the molecule's interior by complexing with K+, forming a bracelet structure, and being surrounded by the hydrophobic exterior conformation. This structure allows a proposed fluorometric method for detecting K+ by switching between the Val-AuNCs' hydrophilicity and hydrophobicity, which induces the aggregation of gold nanoclusters. As a result, significant quenching is seen in fluorescence after adding K+. The quenching in fluorescence in the presence of K+ is attributed to the aggregation mechanism. This sensing technique provides a highly precise and selective sensing method for K+ in the range 0.78 to 8 µM with LOD equal to 233 nM. The selectivity of Val-AuNCs toward K+ ions was investigated compared to other ions. Furthermore, the Val-AuNCs have novel possibilities as favorable sensor candidates for various imaging applications. Our detection technique was validated by determining K+ ions in postmortem vitreous humor samples, which yielded promising results.

Nanoparticle-Based Bioaffinity Assays: From the Research Laboratory to the Market

Advances in the development of new biorecognition elements, nanoparticle-based labels as well as instrumentation have inspired the design of new bioaffinity assays. This review critically discusses the potential of nanoparticles to replace current enzymatic or molecular labels in immunoassays and other bioaffinity assays. Successful implementations of nanoparticles in commercial assays and the need for rapid tests incorporating nanoparticles in different roles such as capture support, signal generation elements, and signal amplification systems are highlighted. The limited number of nanoparticles applied in current commercial assays can be explained by challenges associated with the analysis of real samples (e.g., blood, urine, or nasal swabs) that are difficult to resolve, particularly if the same performance can be achieved more easily by conventional labels. Lateral flow assays that are based on the visual detection of the red-colored line formed by colloidal gold are a notable exception, exemplified by SARS-CoV-2 rapid antigen tests that have moved from initial laboratory testing to widespread market adaption in less than two years.

Optical Nonlinearity Enabled Super-Resolved Multiplexing Microscopy

Optical multiplexing for nanoscale object recognition is of great significance within the intricate domains of biology, medicine, anti-counterfeiting, and microscopic imaging. Traditionally, the multiplexing dimensions of nanoscopy are limited to emission intensity, color, lifetime, and polarization. Here, a novel dimension, optical nonlinearity, is proposed for super-resolved multiplexing microscopy. This optical nonlinearity is attributable to the energy transitions between multiple energy levels of the doped lanthanide ions in upconversion nanoparticles (UCNPs), resulting in unique optical fingerprints for UCNPs with different compositions. A vortex beam is applied to transport the optical nonlinearity onto the imaging point-spread function (PSF), creating a robust super-resolved multiplexing imaging strategy for differentiating UCNPs with distinctive optical nonlinearities. The composition information of the nanoparticles can be retrieved with variations of the corresponding PSF in the obtained image. Four channels multiplexing super-resolved imaging with a single scanning, applying emission color and nonlinearity of two orthogonal imaging dimensions with a spatial resolution higher than 150 nm (1/6.5λ), are demonstrated. This work provides a new and orthogonal dimension - optical nonlinearity - to existing multiplexing dimensions, which shows great potential in bioimaging, anti-counterfeiting, microarray assays, deep tissue multiplexing detection, and high-density data storage.

Communicating Supraparticles to Enable Perceptual, Information-Providing Matter

Materials are the fundament of the physical world, whereas information and its exchange are the centerpieces of the digital world. Their fruitful synergy offers countless opportunities for realizing desired digital transformation processes in the physical world of materials. Yet, to date, a perfect connection between these worlds is missing. From the perspective, this can be achieved by overcoming the paradigm of considering materials as passive objects and turning them into perceptual, information-providing matter. This matter is capable of communicating associated digitally stored information, for example, its origin, fate, and material type as well as its intactness on demand. Herein, the concept of realizing perceptual, information-providing matter by integrating customizable (sub-)micrometer-sized communicating supraparticles (CSPs) is presented. They are assembled from individual nanoparticulate and/or (macro)molecular building blocks with spectrally differentiable signals that are either robust or stimuli-susceptible. Their combination yields functional signal characteristics that provide an identification signature and one or multiple stimuli-recorder features. This enables CSPs to communicate associated digital information on the tagged material and its encountered stimuli histories upon signal readout anywhere across its life cycle. Ultimately, CSPs link the materials and digital worlds with numerous use cases thereof, in particular fostering the transition into an age of sustainability.

Lanthanide-Doped Upconversion Nanoparticles: Exploring A Treasure Trove of NIR-Mediated Emerging Applications

Lanthanide-doped upconversion nanoparticles (UCNPs) possess the remarkable ability to convert multiple near-infrared (NIR) photons into higher energy ultraviolet-visible (UV-vis) photons, making them a prime candidate for several advanced applications within the realm of nanotechnology. Compared to traditional organic fluorophores and quantum dots (QDs), UCNPs possess narrower emission bands (fwhm of 10-50 nm), large anti-Stokes shifts, low toxicity, high chemical stability, and resistance to photobleaching and blinking. In addition, unlike UV-vis excitation, NIR excitation is nondestructive at lower power intensities and has high tissue penetration depths (up to 2 mm) with low autofluorescence and scattering. Together, these properties make UCNPs exceedingly favored for advanced bioanalytical and theranostic applications, where these systems have been well-explored. UCNPs are also well-suited for bioimaging, optically modulating chemistries, forensic science, and other state-of-the-art research applications. In this review, an up-to-date account of emerging applications in UCNP research, beyond bioanalytical and theranostics, are presented including optogenetics, super-resolution imaging, encoded barcodes, fingerprinting, NIR vision, UCNP-assisted photochemical manipulations, optical tweezers, 3D printing, lasing, NIR-II imaging, UCNP-molecule nanohybrids, and UCNP-based persistent luminescent nanocrystals.

Bioconjugates of photon-upconversion nanoparticles for cancer biomarker detection and imaging

The detection of cancer biomarkers in histological samples and blood is of paramount importance for clinical diagnosis. Current methods are limited in terms of sensitivity, hindering early detection of disease. We have overcome the shortcomings of currently available staining and fluorescence labeling methods by taking an integrative approach to establish photon-upconversion nanoparticles (UCNP) as a powerful platform for cancer detection. These nanoparticles are readily synthesized in different sizes to yield efficient and tunable short-wavelength light emission under near-infrared excitation, which eliminates optical background interference of the specimen. Here we present a protocol for the synthesis of UCNPs by high-temperature co-precipitation or seed-mediated growth by thermal decomposition, surface modification by silica or poly(ethylene glycol) that renders the particles resistant to nonspecific binding, and the conjugation of streptavidin or antibodies for biological detection. To detect blood-based biomarkers, we present an upconversion-linked immunosorbent assay for the analog and digital detection of the cancer marker prostate-specific antigen. When applied to immunocytochemistry analysis, UCNPs enable the detection of the breast cancer marker human epidermal growth factor receptor 2 with a signal-to-background ratio 50-fold higher than conventional fluorescent labels. UCNP synthesis takes 4.5 d, the preparation of the antibody-silica-UCNP conjugate takes 3 d, the streptavidin-poly(ethylene glycol)-UCNP conjugate takes 2-3 weeks, upconversion-linked immunosorbent assay takes 2-4 d and immunocytochemistry takes 8-10 h. The procedures can be performed after standard laboratory training in nanomaterials research.

Covert MOF-Based Photoluminescent Tags via Tunable Linker Energetics

Optical anticounterfeiting tags utilize the photoluminescent properties of materials to encode unique patterns, enabling identification and validation of important items and assets. These tags must combine optical complexity with ease of production and authentication to both prevent counterfeiting and to remain practical for widespread use. Metal-organic frameworks (MOFs) based on polynuclear, rare earth clusters are ideal materials platforms for this purpose, combining fine control over structure and composition, with tunable, complex energy transfer mechanisms via both linker and metal components. Here we report the design and synthesis of a set of heterometallic MOFs based on combinations of Eu, Nd, and Yb with the tetratopic linker 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene. The energetics of this linker facilitate the intentional concealment of the visible emissions from Eu while retaining the infrared emissions of Nd and Yb, creating an optical tag with multiple covert elements. Unique to the materials system reported herein, we document the occurrence of a previously not observed 11-metal cluster correlated with the presence of Yb in the MOFs, coexisting with a commonly encountered 9-metal cluster. We demonstrate the utility of these materials as intricate optical tags with both rapid and in-depth screening techniques, utilizing orthogonal identifiers across composition, emission spectra, and emission decay dynamics. This work highlights the important effect of linker selection in controlling the resulting photoluminescent properties in MOFs and opens an avenue for the targeted design of highly complex, multifunctional optical tags.

High Color-Purity Red, Green, and Blue-Emissive Core-Shell Upconversion Nanoparticles Using Ternary Near-Infrared Quadrature Excitations

Development of multicolor-emitting upconversion nanoparticles (UCNPs) is of significant importance for applications in optical encoding, anti-counterfeiting, display, and bioimaging. However, realizing the orthogonal three-primary color (TPC) upconversion luminescence in a single nanoparticle remains a huge challenge. Herein, we have rationally designed core-multishell-structured NaYF₄ UCNPs through regulating the dopant concentration, composition of luminescent layers, and shell position and thickness, which are capable of emitting red, green, and blue luminescence with high color purity in response to ternary near-infrared quadrature excitations (1560/808/980 nm). Moreover, their high color purity is well retained with varying excitation power densities. This orthogonal TPC emissions property of such UCNPs endows them with great promise in the field of security. As a proof-of-concept, we have demonstrated the feasibility of combining such UCNPs with MnO₂ nanosheets for information encryption and decryption. This work not only offers a new way to achieve TPC upconversion luminescence at a single nanoparticle level but also broadens the scope of application for security protection.

Encoding Multilayer Complexity in Anti-Counterfeiting Heterometallic MOF-Based Optical Tags

Optical tags provide a way to quickly and unambiguously identify valuable assets. Current tag fluorophore options lack the tunability to allow combined methods of encoding in a single material. Herein we report a design strategy to encode multilayer complexity in a family of heterometallic rare-earth metal-organic frameworks based on highly connected nonanuclear clusters. To impart both intricacy and security, a synergistic approach was implemented resulting in both overt (visible) and covert (near-infrared, NIR) properties, with concomitant multi-emissive spectra and tunable luminescence lifetimes. Tag authentication is validated with a variety of orthogonal detection methodologies. Importantly, the effect induced by subtle compositional changes on intermetallic energy transfer, and thus on the resulting photophysical properties, is demonstrated. This strategy can be widely implemented to create a large library of highly complex, difficult-to-counterfeit optical tags.

Multiplexed Nanobiosensors: Current Trends in Early Diagnostics

The ever-growing demand for fast, cheap, and reliable diagnostic tools for personalised medicine is encouraging scientists to improve existing technology platforms and to create new methods for the detection and quantification of biomarkers of clinical significance. Simultaneous detection of multiple analytes allows more accurate assessment of changes in biomarker expression and offers the possibility of disease diagnosis at the earliest stages. The concept of multiplexing, where multiple analytes can be detected in a single sample, can be tackled using several types of nanomaterial-based biosensors. Quantum dots are widely used photoluminescent nanoparticles and represent one of the most frequent choices for different multiplex systems. However, nanoparticles that incorporate gold, silver, and rare earth metals with their unique optical properties are an emerging perspective in the multiplexing field. In this review, we summarise progress in various nanoparticle applications for multiplexed biomarkers.

Laminar Flow-Based Fiber Fabrication and Encoding via Two-Photon Lithography

In recent years, flow photolithography (FL) has emerged as a powerful synthetic tool for the creation of barcoded microparticles with complex morphologies and chemical compositions which have been shown to be useful in a range of multiplexed bioassay applications. More specifically, FL has been highly successful in producing micron-sized, encoded particles of bespoke shape, size, and color. That said, to date, FL has been restricted to generating barcoded microparticles and has lacked the ability to produce hybrid fibers which are structurally and spectrally encoded. To this end, we herein present a method that combines a continuous flow microfluidic system with two-photon polymerization (2PP) to fabricate microscale-encoded fibers and Janus strips in a high-throughput manner. Specifically, two co-flow liquid streams containing a monomer and initiator are introduced through a Y-shape channel to form a stable interface in the center of a microfluidic channel. The flow containing the (fluorescently labeled) monomer is then patterned by scanning the voxel of the 2PP laser across the interface to selectively polymerize different regions of the forming fiber/particle. Such a process allows for rapid spectral encoding at the single fiber level, with the resulting structurally coded fibers having obvious application in the fields of security identification and anticounterfeiting.

Prospects on Nano-Based Platforms for Antimicrobial Photodynamic Therapy Against Oral Biofilms

Objective: This review clusters the growing field of nano-based platforms for antimicrobial photodynamic therapy (aPDT) targeting pathogenic oral biofilms and increase interactions between dental researchers and investigators in many related fields. Background data: Clinically relevant disinfection of dental tissues is difficult to achieve with aPDT alone. It has been found that limited penetrability into soft and hard dental tissues, diffusion of the photosensitizers, and the small light absorption coefficient are contributing factors. As a result, the effectiveness of aPDT is reduced in vivo applications. To overcome limitations, nanotechnology has been implied to enhance the penetration and delivery of photosensitizers to target microorganisms and increase the bactericidal effect. Materials and methods: The current literature was screened for the various platforms composed of photosensitizers functionalized with nanoparticles and their enhanced performance against oral pathogenic biofilms. Results: The evidence-based findings from the up-to-date literature were promising to control the onset and the progression of dental biofilm-triggered diseases such as dental caries, endodontic infections, and periodontal diseases. The antimicrobial effects of aPDT with nano-based platforms on oral bacterial disinfection will help to advance the design of combination strategies that increase the rate of complete and durable clinical response in oral infections. Conclusions: There is enthusiasm about the potential of nano-based platforms to treat currently out of the reach pathogenic oral biofilms. Much of the potential exists because these nano-based platforms use unique mechanisms of action that allow us to overcome the challenging of intra-oral and hard-tissue disinfection.

Recent advances of lanthanide-doped upconversion nanoparticles for biological applications

Near infrared (NIR) excited lanthanide-doped upconversion nanoparticles (UCNPs) are emerging as a new type of fluorescent tag for biological applications, which can emit multi-photon ultraviolet, visible or NIR luminescence for imaging or activation of photosensitive molecules. Here, we present a comprehensive review on recent advances of UCNPs for a manifold of biological applications, including upconversion mechanisms, building bright multicolor upconversion nanocrystals, single nanoparticle and super resolution imaging, in vivo optical and multimodal imaging, photodynamic therapy, light-controlled drug release, biosensing, and toxicities. Our perspectives on the future development of UCNPs are also described.

Organic micro/nanoscale materials for photonic barcodes

This review summarizes recent advances in micro/nanoscale photonic barcodes based on organic materials from the aspects of diverse optical encoding techniques.

Photon-Upconversion Barcoding with Multiple Barcode Channels: Application for Droplet Microfluidics

Barcoding facilitates high-throughput analytical methods in complex matrixes with a reduced volume of sample, reagents, time, and cost. Because of orthogonality to fluorescence, photon-upconversion barcodes attracted considerable attention in recent years. We constructed an epiluminescence detector, which, for the first time, demonstrated the reading of photon-upconversion spectra from microdroplets in a microfluidic chip with frequency up to 10 Hz. Non-negative least-squares deconvolution enabled the reading of an unprecedented number of photon-upconversion barcode channels (six) from emission spectra (excitation 980 nm, emission 430-875 nm). The standard deviation of barcode reading from microdroplets was ∼1%. Described barcoding can be, for example, used for multiparameter titrations, multiplexed biological and chemical assays, optimizations on a microfluidic platform, and preparation of barcoded concentration gradients and libraries.

Aggregation-induced heterogeneities in the emission of upconverting nanoparticles at the submicron scale unfolded by hyperspectral microscopy

Transparent upconverting hybrid nanocomposites are exciting materials for advanced applications such as 3D displays, nanosensors, solar energy converters, and fluorescence biomarkers. This work presents a simple strategy to disperse upconverting β-NaYF4:Yb3+/Er3+ or Tm3+ nanoparticles into low cost, widely used and easy-to-process polydimethylsiloxane (PDMS)-based organic-inorganic hybrids. The upconverting hybrids were shaped as monoliths, films or powders displaying in the whole volume Tm3+ or Er3+ emissions (in the violet/blue and green/red spectral regions, respectively). For the first time, hyperspectral microscopy allows the identification at the submicron scale of differences in the hybrids' emission colour, due to variations in the relative intensity of the distinct components of the upconversion spectrum. The effect is attributed to the size distribution of the agglomerates of nanoparticles, highlighting the importance of studying the emission at submicron scales, since this effect is not observable in measurements recorded in larger areas.

Precisely tailored shell thickness and Ln3+ content to produce multicolor emission from Nd3+-sensitized Gd3+-based core/shell/shell UCNPs through bi-directional energy transfer

Lanthanide (Ln³⁺)-doped upconversion nanoparticles (UCNPs) have been paid great attention as multiplexing agents due to their numerous uses in biological and clinical applications such as bioimaging and magnetic resonance imaging (MRI), to name a few. To achieve efficient multicolor emission from UCNPs under single 808 nm excitation and avoid detrimental cross-relaxations between the Ln³⁺ activator ions (positioned in either the core and/or shell in the core/shell), it is essential to design an adequate nanoparticle architecture. Herein, we demonstrate the tailoring of multicolor upconversion luminescence (UCL) from Nd³⁺-sensitized Gd³⁺-based core/shell/shell UCNPs with an architecture represented as NaGdF₄:Tm³⁺(0.75)/Yb³⁺(40)/Ca²⁺(7)/Nd³⁺(1)@NaGdF₄:Ca²⁺(7)/Nd³⁺(30)@NaGdF₄:Yb³⁺(40)/Ca²⁺(7)/Nd³⁺(1)/Er³⁺(X = 1, 2, 3, 5, 7) [hereafter named CSS (Er³⁺ = 1, 2, 3, 5 and 7 mol%)]. Such UCNPs can be excited at a single wavelength (∼808 nm) without generation of any local heat. Incorporation of substantial Nd³⁺-sensitizers with an appropriate concentration in the middle layer allows efficient harvesting of excitation light which migrates bi-directionally across the core/shell interfaces in sync to produce blue emission from Tm³⁺ (activator) ions in the core as well as green and red emission from Er³⁺ (activator) ions in the outermost shell. Introduction of Ca²⁺ lowers the local crystal field symmetry around Ln³⁺ ions and subsequently affects their intra 4f-4f transition probability, thus enhancing the upconversion efficiency of the UCNPs. By simple and precise control of the shell thickness along with tuning the content of Ln³⁺ ions in each domain, multicolor UCL can be produced, ranging from blue to white. We envision that our sub-20 nm sized Nd³⁺-sensitized Gd³⁺-based UCNPs are not only potential candidates for a variety of multiplexed biological applications (without impediment of any heating effect), but also can act as MRI contrast agents in clinical diagnosis.

Dual-modal imaging and excellent anticancer efficiency of cisplatin and doxorubicin loaded NaGdF4:Yb3+/Er3+ nanoparticles

NaGdF₄:Yb³⁺/Er³⁺ nanoparticles were synthesized via a modified hydrothermal route. The dependence of structure and morphology on the dosage of sodium polyacrylate was studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The as-prepared nanoparticles could be used for T₂ weighted magnetic resonance imaging due to the paramagnetism of Gd³⁺. cis-dichlorodiamineplatinum (CDDP) could be loaded onto NaGdF₄:Yb³⁺/Er³⁺ nanoparticles through binding carboxyl in the form of Pt–O bonds, and doxorubicin (DOX) could be loaded via hydrogen bonding. DOX could also be loaded onto the NaGdF₄–CDDP composite in the same manner, and the loading efficiency of both drugs remained unchanged. Three as-prepared drug delivery systems were used for tumor inhibition both in vitro and in vivo, and the results indicated that NaGdF₄–CDDP–DOX displayed the greatest inhibitory capacity.

The drug delivery system NaGdF₄–CDDP–DOX showed best tumor inhibition capacity both in vitro and in vivo.

Energy Migration Upconversion in Ce(III)-Doped Heterogeneous Core-Shell-Shell Nanoparticles

One major challenge in upconversion research is to develop new materials and structures to expand the emission spectrum. Herein, a heterogeneous core-shell-shell nanostructure of NaYbF₄ :Gd/Tm@NaGdF₄ @CaF₂ :Ce is developed to realize efficient photon upconversion in Ce³⁺ ions through a Gd-mediated energy migration process. The design takes advantage of CaF₂ host that reduces the 4f-5d excitation frequency of Ce³⁺ to match the emission line of Gd³⁺ . Meanwhile, CaF₂ is isostructural with NaGdF₄ and can form a continuous crystalline lattice with the core layer. As a result, effective Yb³⁺ → Tm³⁺ → Gd³⁺ → Ce³⁺ energy transfer can be established in a single nanoparticle. This effect enables efficient ultraviolet emission of Ce³⁺ following near infrared excitation into the core layer. The Ce³⁺ upconversion emission achieved in the core-shell-shell nanoparticles features broad bandwidth and long lifetime, which offers exciting opportunities of realizing tunable lasing emissions in the ultraviolet spectral region.

Covert Photonic Barcodes Based on Light Controlled Acidichromism in Organic Dye Doped Whispering-Gallery-Mode Microdisks

Photonic barcodes with a small footprint have demonstrated a great value for multiplexed high-throughput bioassays and tracking systems. Attempts to develop coding technology tend to focus on the generation of featured barcodes both with high coding capacity and accurate recognition. In this work, a strategy to design photonic barcodes is proposed based on whispering-gallery-mode (WGM) modulations in dye-doped microdisk resonant cavities, where each modulated photoluminescence spectrum constitutes the fingerprint of a corresponding microdisk. The WGM-based barcodes can achieve infinite encoding capacity through tuning the dimensions of the microdisks. These photonic barcodes can be well disguised and decoded based on the light controlled proton release and acidichromism of the organic materials, which are essential to fulfill the functions of anti-counterfeiting, information security, and so on. The results will pave an avenue to new types of flexible WGM-based components for optical data recording and security labels.

UV and Temperature-Sensing Based on NaGdF4:Yb3+:Er3+@SiO2-Eu(tta)3

A multifunctional nanosystem was synthesized to be used as a dual sensor of UV light and temperature. NaGdF4:Yb3+:Er3+ upconverting nanoparticles (UCNPs) were synthesized and coated with a silica shell to which a europium(III) complex was incorporated. The synthesis of NaGdF4 UCNPs was performed via thermal decomposition of lanthanide ion fluoride precursors in the presence of oleic acid. To achieve sufficient water dispersibility, the surface of the hydrophobic oleate-capped UCNPs in the hexagonal phase was modified by a silica coating through a modified Stöber process through a reverse microemulsion method. An Eu(tta)3 (tta: thenoyltrifluoroacetonate) complex was prepared in situ at the silica shell. A dual-mode nanothermometer was obtained from a near infrared to visible upconversion fluorescence signal of Er3+ ions together with UV-excited downshifting emission from the Eu3+ complex. Measurements were recorded near the physiological temperature range (293-323 K), revealing excellent linearity (R 2 > 0.99) and relatively high thermal sensitivities (≥1.5%·K-1). The Eu(tta)3 complex present in the silica shell was tested as the UV sensor because of the Eu3+ luminescence dependence on UV-light exposure time.

Tuning the upconversion light emission by bandgap engineering in bismuth oxide-based upconverting nanoparticles

In the field of novel applications involving upconverting processes, the determination of new strategies for realizing emission-tunable nanomaterials is a challenge. In this work the design of Y³⁺ and Er³⁺ codoped bismuth oxide-based upconverting nanoparticles is presented, evidencing that the active role of the matrix allows for the emission selectivity with chromaticity control. The bandgap of the bismuth oxide-based host can be manipulated in the range of 0.65 eV, consequently leading to upconversion emission color tunability from red to yellow-greenish. The resulting fine control of the nanoparticle chromaticity through accurate host bandgap engineering reveals a novel concept for the development of a new generation of upconverting nanophosphors.

Pyrolyzable Nanoparticle Tracers for Environmental Interrogation and Monitoring

Environmental tracing applications require materials that can be detected in complex fluids composed of multiple phases and contaminants. Moreover, large libraries of tracers are necessary in order to mitigate memory effects and to deploy multiple tracers simultaneously in complex oil fields. Herein, we disclose a novel approach based on the thermal decomposition of polymeric nanoparticles comprised of styrenic and methacrylic monomers. Polymeric nanoparticles derived from these monomers cleanly decompose into their constituent monomers at elevated temperatures, thereby maximizing atom economy wherein the entire nanoparticle mass contributes to the generation of detectable units. A total of ten unique single monomer particles and three dual-monomer particles were synthesized using semicontinuous monomer starved addition polymerization. The pyrolysis gas chromatography-flame ionization detection/mass spectrometry (GC-FID/MS) behavior of these particles was studied using high-pressure mass spectrometry. The programmable nature of our methodology permits simultaneous removal of contaminants and subsequent identification and quantification in a single analytical step.

Programmable Microfluidic Synthesis of Over One Thousand Uniquely Identifiable Spectral Codes

Encoded microparticles have become a powerful tool for a wide array of applications, including high-throughput sample tracking and massively parallel biological multiplexing. Spectral encoding, where particles are encoded with distinct luminescence spectra, provides a particularly appealing encoding strategy because of the ease of reading codes and assay flexibility. To date, spectral encoding has been limited in the number of codes that can be accurately resolved. Here, we demonstrate an automated 5-dimensional spectral encoding scheme using lanthanide nanophosphors that is capable of producing isotropic spherical microparticles with up to 1,100 unique codes, which we term MRBLEs (Microspheres with Ratiometric Barcode Lanthanide Encoding). We further develop a quantitative framework for evaluating global ability to distinguish codes and demonstrate that for six different sets of MRBLEs ranging from 106 to 1,101 codes in size, > 98% of MRBLEs can be assigned to a code with 99.99% confidence. These > 1,000 code sets represent the largest spectral code libraries built to date. We expect that these MRBLEs will enable a wide variety of novel multiplexed assays.

Resonance Energy Transfer in Upconversion Nanoplatforms for Selective Biodetection

Resonance energy transfer (RET) describes the process that energy is transferred from an excited donor to an acceptor molecule, leading to a reduction in the fluorescence emission intensity of the donor and an increase in that of the acceptor. By this technique, measurements with the good sensitivity can be made about distance within 1 to 10 nm under physiological conditions. For this reason, the RET technique has been widely used in polymer science, biochemistry, and structural biology. Recently, a number of RET systems incorporated with nanoparticles, such as quantum dots, gold nanoparticles, and upconversion nanoparticles, have been developed. These nanocrystals retain their optical superiority and can act as either a donor or a quencher, thereby enhancing the performance of RET systems and providing more opportunities in excitation wavelength selection. Notably, lanthanide-doped upconversion nanophosphors (UCNPs) have attracted considerable attention due to their inherent advantages of large anti-Stoke shifts, long luminescence lifetimes, and absence of autofluorescence under low energy near-infrared (NIR) light excitation. These nanoparticles are promising for the biodetection of various types of analytes. Undoubtedly, the developments of those applications usually rely on resonance energy transfer, which could be regarded as a flexible technology to mediate energy transfer from upconversion phosphor to acceptor for the design of luminescent functional nanoplatforms. Currently, researchers have developed many RET-based upconversion nanosystems (RET-UCNP) that respond to specific changes in the biological environments. Specifically, small organic molecules, biological molecules, metal-organic complexes, or inorganic nanoparticles were carefully selected and bound to the surface of upconversion nanoparticles for the preparation of RET-UCNP nanosystems. Benefiting from the advantage and versatility offered by this technology, the research of RET-based upconversion nanomaterials should have significant implications for advanced biomedical applications. It should be noted that energy transfer in a UCNP based nanosystem is most often related to resonance energy transfer but that reabsorption (and maybe other energy transfer processes) may also play an important role and that more studies regarding the fundamental aspects for energy transfer with UCNPs is necessary. In this Account, we present an overview of recent advances in RET-based upconversion nanocomposites for biodetection with a particular focus on our own work. We have designed a series of upconversion nanoplatforms with remarkably high versatility for different applications. The experience gained from our strategic design and experimental investigations will allow for the construction of next-generation luminescent nanoplatform with marked improvements in their performance. The key aspects of this Account include fundamental principles, design and preparation strategies, biodetection in vitro and in vivo, future opportunities, and challenges of RET-UCNP nanosystems.

Assessing inter lanthanide photophysical interactions in co-doped titanium dioxide nanoparticles for multiplex assays

This work assesses inter lanthanide photophysical interactions in titanium dioxide nanoparticles towards the development of multiplex assays.

New advances on the marrying of UCNPs and photothermal agents for imaging-guided diagnosis and the therapy of tumors

This review primarily focuses on the new advances in the design and theranostic applications of rare earth upconversion nanoparticles (UCNPs)–NIR photothermal absorbers multifunctional nanoplatforms.

Amplifying Excitation-Power Sensitivity of Photon Upconversion in a NaYbF4:Ho Nanostructure for Direct Visualization of Electromagnetic Hotspots

Controlling excitation power is the most convenient approach to dynamically tuning upconversion that is essential for a variety of studies. However, this approach suffers from a significant constraint due to insensitive response of most upconversion systems to excitation power. Here we present a study of amplifying excitation power-sensitivity of upconversion in Ho³⁺ ions through the use of a NaYbF₄ host. Mechanistic investigation reveals that the sensitive response of Ho³⁺ upconversion to excitation power stems from maximal use of the incident energy enabled by concentrated Yb³⁺ sensitizers. This allows us to sensitively tune the red-to-green emission intensity ratio from 0.37 to 5.19 by increasing the excitation power from 1.25 to 46.25 W cm^-2, which represents a 5.6-fold amplification of the tunability (from 0.19 to 0.49) offered by Yb/Ho (19/1 mol %) codoped NaYF₄. Our results highlight that the excitation-power sensitive upconversion emission can be exploited to experimentally visualize electromagnetic hotspots.

Competitive Upconversion-Linked Immunosorbent Assay for the Sensitive Detection of Diclofenac

Photon-upconverting nanoparticles (UCNPs) emit light of shorter wavelength under near-infrared excitation and thus avoid optical background interference. We have exploited this unique photophysical feature to establish a sensitive competitive immunoassay for the detection of the pharmaceutical micropollutant diclofenac (DCF) in water. The so-called upconversion-linked immunosorbent assay (ULISA) was critically dependent on the design of the upconversion luminescent detection label. Silica-coated UCNPs (50 nm in diameter) exposing carboxyl groups on the surface were conjugated to a secondary anti-IgG antibody. We investigated the structure and monodispersity of the nanoconjugates in detail. Using a highly affine anti-DCF primary antibody, the optimized ULISA reached a detection limit of 0.05 ng DCF per mL. This performance came close to a conventional enzyme-linked immunosorbent assay (ELISA) without the need for an enzyme-mediated signal amplification step. The ULISA was further employed for analyzing drinking and surface water samples. The results were consistent with a conventional ELISA as well as liquid chromatography-mass spectrometry (LC-MS).

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.

Engineering of Lanthanide-Doped Upconversion Nanoparticles for Optical Encoding

Lanthanide-doped upconversion nanoparticles (UCNPs) are an emerging class of luminescent materials that emit UV or visible light under near infra-red (NIR) excitations, thereby possessing a large anti-Stokes shift property. Due to their sharp excitation and emission bands, excellent photo- and chemical stability, low autofluorescence, and high tissue penetration depth of the NIR light used for excitation, UCNPs have surpassed conventional fluorophores in many bioapplications. A better understanding of the mechanism of upconversion, as well as the development of better approaches to preparing UCNPs, have provided more opportunities to explore their use for optical encoding, which has the potential for applications in multiplex detection and imaging. With the current ability to precisely control the microstructure and properties of UCNPs to produce particles of tunable emission, excitation, luminescence lifetime, and size, various strategies for optical encoding based on UCNPs can now be developed. These optical properties of UCNPs (such as emission and excitation wavelengths, ratiometric intensity, luminescence lifetime, and multicolor patterns), and the strategies employed to engineer these properties for optical encoding of UCNPs through homogeneous ion doping, heterogeneous structure fabrication and microbead encapsulation are reviewed. The challenges and potential solutions faced by UCNP optical encoding are also discussed.

Analysis of Carbohydrate-Carbohydrate Interactions Using Sugar-Functionalized Silicon Nanoparticles for Cell Imaging

Protein-carbohydrate binding depends on multivalent ligand display that is even more important for low affinity carbohydrate-carbohydrate interactions. Detection and analysis of these low affinity multivalent binding events are technically challenging. We describe the synthesis of dual-fluorescent sugar-capped silicon nanoparticles that proved to be an attractive tool for the analysis of low affinity interactions. These ultrasmall NPs with sizes of around 4 nm can be used for NMR quantification of coupled sugars. The silicon nanoparticles are employed to measure the interaction between the cancer-associated glycosphingolipids GM3 and Gg3 and the associated kD value by surface plasmon resonance experiments. Cell binding studies, to investigate the biological relevance of these carbohydrate-carbohydrate interactions, also benefit from these fluorescent sugar-capped nanoparticles.

Simple synthesis of amino acid-functionalized hydrophilic upconversion nanoparticles capped with both carboxyl and amino groups for bimodal imaging

Amino acid-functionalized hydrophilic upconversion nanoparticles capped with both carboxyl and amino groups were one-step synthesized for bimodal imaging.

Surface modification and characterization of photon-upconverting nanoparticles for bioanalytical applications

Photon-upconverting nanoparticles (UCNPs) can be excited by near-infrared light and emit visible light (anti-Stokes emission) which prevents autofluorescence and light scattering of biological samples. The potential for background-free imaging has attracted wide interest in UCNPs in recent years. Small and homogeneous lanthanide-doped UCNPs that display high upconversion efficiency have typically been synthesized in organic solvents. Bioanalytical applications, however, require a subsequent phase transfer to aqueous solutions. Hence, the surface properties of UCNPs must be well designed and characterized to grant both a stable aqueous colloidal dispersion and the ability to conjugate biomolecules and other ligands on the nanoparticle surface. In this review, we introduce various routes for the surface modification of UCNPs and critically discuss their advantages and disadvantages. The last part covers various analytical methods that enable a thorough examination of the progress and success of the surface functionalization.