Upconversion nanoparticles in biological labeling, imaging, and therapy

Compact and Filter-Free Luminescence Biosensor for Mobile in Vitro Diagnoses

We report a sensitive and versatile biosensing approach, LUCID (luminescence compact in vitro diagnostics), for quantitative molecular and cellular analyses. LUCID uses upconversion nanoparticles (UCNPs) as luminescent reporters in mutually exclusive photoexcitation and read-out sequences implemented on a smartphone. The strategy improves imaging signal-to-noise ratios, eliminating interference from excitation sources and minimizing autofluorescence, and thus enables filterless imaging. Here we developed a miniaturized detection system and optimized UCNPs for the system and biological applications. Nanoparticle luminescence lifetime was extended by controlling particle structure and composition. When tested with a range of biological targets, LUCID achieved high detection sensitivity (0.5 pM for protein and 0.1 pM for nucleic acids), differentiated bacterial samples, and allowed profiling of cells. In proof-of-concept clinical use, LUCID demonstrated effective screening of cancer cells in cervical brushing specimens, identifying patients at high risk for malignancy. These results suggest that LUCID could serve as a broadly applicable and inexpensive diagnostic platform.

Optogenetics sheds new light on tissue engineering and regenerative medicine

Optogenetics has demonstrated great potential in the fields of tissue engineering and regenerative medicine, from basic research to clinical applications. Spatiotemporal encoding during individual development has been widely identified and is considered a novel strategy for regeneration. A as a noninvasive method with high spatiotemporal resolution, optogenetics are suitable for this strategy. In this review, we discuss roles of dynamic signal coding in cell physiology and embryonic development. Several optogenetic systems are introduced as ideal optogenetic tools, and their features are compared. In addition, potential applications of optogenetics for tissue engineering are discussed, including light-controlled genetic engineering and regulation of signaling pathways. Furthermore, we present how emerging biomaterials and photoelectric technologies have greatly promoted the clinical application of optogenetics and inspired new concepts for optically controlled therapies. Our summation of currently available data conclusively demonstrates that optogenetic tools are a promising method for elucidating and simulating developmental processes, thus providing vast prospects for tissue engineering and regenerative medicine applications.

Two-photon based pulse autocorrelation with CdSe nanoplatelets

We investigate broadband two-photon absorption autocorrelators based on II-VI semiconductor nanoplatelets as an alternative to common second harmonic generation based techniques. As compared to bulk materials the exceptionally high enhancement of two-photon absorption in these 2D structures results in very efficient two-photon absorption based autocorrelation detected via PL emission. We compare the results with TPA autocorrelation in CdS bulk as well as SHG based autocorrelation in β-barium borate. We show that CdSe nanoplatelet based autocorrelation can exceed the efficiency of conventional methods by two orders in magnitude, especially for short interaction length, and allows a precise pulse-width determination. We demonstrate that very high two-photon absorption cross sections of the nanoplatelets are the basis for this effective TPA autocorrelation. Based on our results with II-VI nanoplatelets efficient broadband autocorrelation with more than ∼100 nm bandwidth and very high sensitivity seems feasible.

Rhodamine B derivative-modified up-conversion nanoparticle probes based on fluorescence resonance energy transfer (FRET) for the solid-based detection of copper ions

Herein, a novel solid-based up-conversion fluorescence resonance energy transfer (FRET) sensor was developed using rhodamine B hydrazide, which provided a selective fluorescence response and suitable affinity towards Cu²⁺ ions over other biologically relevant metal ions because the Cu²⁺ ion could promote the hydrolysis of α-amino acid esters of rhodamine B hydrazide and yield the Cu·α-amino acid chelate. This solid-based detection system is more convenient for the detection of Cu²⁺ based on color change and emission spectra instead of the complicated and tedious measurements than other up-conversion sensors and up-conversion luminescent nanoparticles used as an excitation source; moreover, the proposed system shows high selectivity, minimum photo-damage to living organisms, and high chemical stability.

Near-Infrared Excited Orthogonal Emissive Upconversion Nanoparticles for Imaging-Guided On-Demand Therapy

Photodynamic therapy (PDT) has been considered as a promising and noninvasive strategy for clinical cancer treatment. Nonetheless, building a smart "off-on" theranostic PDT platform to spatiotemporally control the generation of reactive oxygen species in the PDT treatment still remains challenging. Here, we have rationally developed photoswitching upconversion nanoparticles (UCNPs) with orthogonal emissive properties in response to two distinct near-infrared (NIR) emissions at 808 and 980 nm, i.e., red emission with 980 nm excitation and green emission with 808 nm excitation. Unlike traditional photoswitching UCNPs, these specially designed core-shell-shell structured UCNPs do not require complicated multilayer doping as their red and green upconversion luminescence both originate from the same activator Er³⁺ ions in the core structure. As a proof of concept, we have demonstrated the capability of these orthogonal emissive UCNPs for imaging-guided PDT in a real-time manner, where the red emission excited by 980 nm light is used to trigger PDT and the green emission with 808 nm excitation is to diagnose and monitor the therapeutic treatment. Our study suggests that such specially designed UCNPs with orthogonal emissions hold great promise for NIR light-targeted and imaging-guided therapy under precisely spatiotemporal control.

Breakthroughs in medicine and bioimaging with up-conversion nanoparticles

Nanomedicine is a medical application of biochemistry incorporated with materials chemistry at the scale of nanometer for the purpose of diagnosis, prevention, and treatment. New models and approaches are typically associated with nanomedicine for precise multifunctional diagnostic systems at molecular level. Hence, employing nanoparticles (NPs) has unveiled new opportunities for efficient therapies and remedy of difficult-to-cure diseases. Among all types of inorganic NPs, lanthanide-doped up-conversion nanoparticles (UCNPs) have shown excellent potential for biomedical applications, especially for multimodal bioimaging including fluorescence and electron microscopy. Association of these visualization techniques plus the capability for transporting biomaterials and drugs make them superior agents in the field of nanomedicine. Accordingly, in this review, we firstly presented a fundamental understanding of physical and optical properties of UCNPs and secondly, we illustrated some of the prominent associations with bioimaging, theranostics, cancer therapy, and optogenetics.

Toward photophysical characteristics of triplet-triplet annihilation photon upconversion: a promising protocol from the perspective of optimally tuned range-separated hybrids

The photon upconversion (UC) process assisted by the triplet-triplet annihilation (TTA) mechanism has recently come into the spotlight. Given the rich collection of efforts in this area, theoretical explorations regarding TTA-UC are relatively limited and have proven to be challenging for its control in devices. In this contribution, the photophysical properties crucial for TTA-UC, such as triplet excited state energies and triplet-triplet energy transfer gaps of the essential ingredients involved in the process, namely sensitizers, annihilators and their pairs, have theoretically been investigated using optimally tuned range-separated hybrid functionals (OT-RSHs) and their screened exchange counterparts, OT-SRSHs. Taking a series of experimentally proven-to-work sensitizer/annihilator pairs as working models, we have constructed and validated several variants of OT-RSHs using both full time-dependent and Tamm-Dancoff formalisms for a reliable description of the TTA-UC photophysics. Given the bimolecular biphotonic nature of the TTA-UC process under study, particular attention is paid to the influence of the factors like the underlying density functional approximations and the tunable parameters such as short- and long-range exact-like exchanges as well as the range-separation parameter for both the sensitizers and annihilators separately. Dissecting all the aspects and relying on the appropriate choices from the tested models, we propose an OT-RSH with the correct asymptotic behavior as a cost-effective yet useful tool for this purpose. Not only against the standard RSHs but also in comparison to the conventional hybrids, the newly developed OT-RSH yields a more reliable description for the TTA-UC energetics in the gas phase and dielectric medium. Accountability of the proposed model has further been confirmed for several theoretically designed sensitizer/annihilator pairs prone to be used in the TTA-UC process. Summing up, in light of this study additional pieces of convincing evidence on the quality of OT-(S)RSHs for computational modeling and experimental verifications of the photophysics of the photon UC based on TTA and other possible technologies are showcased.

3D sub-diffraction imaging in a conventional confocal configuration by exploiting super-linear emitters

Sub-diffraction microscopy enables bio-imaging with unprecedented clarity. However, most super-resolution methods require complex, costly purpose-built systems, involve image post-processing and struggle with sub-diffraction imaging in 3D. Here, we realize a conceptually different super-resolution approach which circumvents these limitations and enables 3D sub-diffraction imaging on conventional confocal microscopes. We refer to it as super-linear excitation-emission (SEE) microscopy, as it relies on markers with super-linear dependence of the emission on the excitation power. Super-linear markers proposed here are upconversion nanoparticles of NaYF4, doped with 20% Yb and unconventionally high 8% Tm, which are conveniently excited in the near-infrared biological window. We develop a computational framework calculating the 3D resolution for any viable scanning beam shape and excitation-emission probe profile. Imaging of colominic acid-coated upconversion nanoparticles endocytosed by neuronal cells, at resolutions twice better than the diffraction limit both in lateral and axial directions, illustrates the applicability of SEE microscopy for sub-cellular biology.

A flexible smart membrane consisting of GO composite fibres and upconversion MSNs for microRNA detection

We have developed a photoluminescent membrane for microRNA detection, consisting of chemically modified mesoporous silica nanoparticles (CaF2:Yb/Ho@MSNs) attached, via single stranded DNA probes, to flexible polyurethane fibres coated with graphene oxide (GO). By detecting the release of the luminescent nanoparticles resulting from complementary co-hybridization between target miRNA sequences and the DNA probe, accurate measurements of the miRNA concentration at high sensitivity levels can be obtained. The constructs therefore offer a route to rapid detection and the potential for early cancer diagnosis.

Optically Robust and Biocompatible Mechanosensitive Upconverting Nanoparticles

Upconverting nanoparticles (UCNPs) are promising tools for background-free imaging and sensing. However, their usefulness for in vivo applications depends on their biocompatibility, which we define by their optical performance in biological environments and their toxicity in living organisms. For UCNPs with a ratiometric color response to mechanical stress, consistent emission intensity and color are desired for the particles under nonmechanical stimuli. Here, we test the biocompatibility and mechanosensitivity of α-NaYF4:Yb,Er@NaLuF4 nanoparticles. First, we ligand-strip these particles to render them dispersible in aqueous media. Then, we characterize their mechanosensitivity (∼30% in the red-to-green spectral ratio per GPa), which is nearly 3-fold greater than those coated in oleic acid. We next design a suite of ex vivo and in vivo tests to investigate their structural and optical properties under several biorelevant conditions: over time in various buffers types, as a function of pH, and in vivo along the digestive tract of Caenorhabditis elegans worms. Finally, to ensure that the particles do not perturb biological function in C. elegans, we assess the chronic toxicity of nanoparticle ingestion using a reproductive brood assay. In these ways, we determine that mechanosensitive UCNPs are biocompatible, i.e., optically robust and nontoxic, for use as in vivo sensors to study animal digestion.

Potential Application of Upconverting Nanoparticles for Brain Photobiomodulation

Brain photobiomodulation (PBM) describes the use of visible to near-infrared light for modulation or stimulation of the central nervous system in both healthy individuals and diseased conditions. Although the transcranial approach to delivering light to the head is the most common technique to stimulate the brain, delivery of light to deeper structures in the brain is still a challenge. The science of nanoparticle engineering in combination with biophotonic excitation could provide a way to overcome this problem. Upconversion is an anti-Stokes process that is capable of transforming low energy photons that penetrate tissue well to higher energy photons with a greater biological effect, but poor tissue penetration. Wavelengths in the third optical window are optimal for light penetration into brain tissue, followed by windows II, IV, and I. The combination of trivalent lanthanide ions within a crystalline host provides a nanostructure that exhibits the upconversion phenomenon. Upconverting nanoparticles (UCNPs) have been successfully used in various medical fields. Their ability to cross the brain-blood barrier and their low toxicity make them a good candidate for application in brain disorders. It is possible that delivery of UCNPs to the brainstem or deeper parts of the cerebral tissue, followed by irradiation using light wavelengths with good tissue penetration properties, could allow more efficient PBM of the brain.

Improved Red Emission and Short-Wavelength Infrared Luminescence under 808 nm Laser for Tumor Theranostics

In this research, a multimodal imaging platform guided photodynamic theranostics under 808 nm was designed using a NaErF₄:Tm@NaYF₄:Yb@NaLuF₄:Nd,Yb-ZnPc structure. Unlike conventional codoped Yb³⁺/Er³⁺ system, Er³⁺ ions as activator and sensitizer were used to improve the up-conversion energy transfer processes. Furthermore, higher energy transfer processes between Er³⁺ ions could be obtained through doped 1% Tm³⁺ ions as an energy trapping center in the NaErF₄. This platform could emit much brighter upconversion luminescence (UCL) (124-fold enhancement for red emission) and short wavelength infrared (SWIR) emission under single 808 nm laser excitation. Importantly, the SWIR imaging with higher resolution and better signal-to-noise ratio can pass the blood-brain barrier to see the brain vessels. Because of the enhanced red emission, the UCL nanoparticles were combined with ZnPc agent to exhibit photodynamic therapy (PDT) effect, and its distribution and excretion could be detected by the photoacoustic (PA) imaging under single near-infrared (NIR) laser. Thus, this platform could be used as multimodal imaging (SWIR, PA, CT, and UCL) guided PDT agent under single 808 nm laser.

Multicolor lanthanide-doped CaS and SrS near-infrared stimulated luminescent nanoparticles with bright emission: application in broad-spectrum lighting, information coding, and bio-imaging

A series of lanthanide-doped CaS and SrS nanoparticles possessing bright fluorescence under near-infrared light (NIR) illumination and tunable emission colors are prepared and applied in light emitting, information coding, cell-labeling and in vivo imaging. The high fluorescence intensity and easily tunable emission wavelength under 980 nm light excitation make these multicolor alkaline earth sulfide nanoparticles (AES NPs) good in vivo biolabels for simultaneous differentiation of multiple organs, and other NIR-excitation-based applications.

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.

Polymerization Assisted by Upconversion Nanoparticles under NIR Light

Photopolymerization of nanocomposite materials using near infrared light is one of the unique technologies based on the luminescent properties of lanthanide-doped upconversion nanoparticles (UCNPs). We explored the UCNP-triggered radical polymerization both in oligomer bulk and on the nanoparticle surface in aqueous dispersion. Core/shell UCNPs NaYF₄:Yb³⁺ and Tm³⁺/NaYF₄ with emitting lines in the ultraviolet and blue regions were used to activate a photoinitiator. The study of the bulk photopolymerization in an initially homogeneous reaction mixture showed the UCNP redistribution due to gradient density occurring in the volume, which led to formation of UCNP superlattices and spheres "frozen" in a polymer matrix. We also developed a strategy of "grafting from" the surface, providing polymer shell growth directly on the nanoparticles. The photosensitization of the endogenous water-soluble photoinitiator riboflavin by the resonance energy transfer from UCNPs was demonstrated in the course of monomer glycidyl methacrylate polymerization followed by photocrosslinking with poly(ethylene glycol) diacrylate on the nanoparticle surface.

Optically Active Hybrid Materials Based on Natural Spider Silk

Spider silk is a natural material possessing unique properties such as biocompatibility, regenerative and antimicrobial activity, and biodegradability. It is broadly considered an attractive matrix for tissue regeneration applications. Optical monitoring and potential control over tissue regrowth are attractive tools for monitoring of this process. In this work, we show upconversion modification of natural spider silk fibers with inorganic nanoparticles. To achieve upconversion, metal oxide nanoparticles were doped with low concentrations of rare-earth elements, producing potentially biocompatible luminescent nanomaterials. The suggested approach to spider silk modification is efficient and easy to perform, opening up sensing and imaging possibilities of biomaterials in a noninvasive and real-time manner in bio-integration approaches.

Cupredoxin engineered upconversion nanoparticles for ratiometric luminescence sensing of Cu2

The NIR excitation and large anti-Stokes shift of upconversion nanoparticles (UCNPs) have made them an ideal choice as biological nanoprobes. A key challenge in the field is to confer biorecognition units to UCNPs so that they can be used to probe specific targets in biological systems. While various agents have been combined with UCNPs to meet such a challenge, most studies are limited to small molecules, while biomolecules such as metalloproteins that possess much higher affinity and selectivity for metal ions have not been explored. Herein we demonstrate that fusion of zwitterion-coated UCNPs with azurin, a member of a family of redox-active copper proteins called cupredoxins that play important roles in diverse biological functions, can serve as an ideal platform for the label-free upconversion luminescence sensing of Cu2+ with a ratiometric response. The selectively binding of apo-azurin with Cu2+ induce a significant absorbance at about 625 nm, and hence decreases the red emission of the UCNPs. In contrast, the green emission of the UCNPs remains constant and acts as an internal standard reference for the ratiometric sensing of Cu2+. This approach opens a new window for the development of assays for biosensing based on a combination of specific metalloproteins with UCNPs.

The role of an inert shell in improving energy utilization in lanthanide-doped upconversion nanoparticles

Coating with an inert shell is considered an efficient approach for enhancing the emission and improving the luminescence quantum yield of lanthanide-doped upconversion nanoparticles. In this work, the role of an inert shell in the photon energy collection and energy transfer behavior is carefully studied. Our experimental results indicate that, owing to the valid energy trapping by the shell, an energy clustering effect is formed among the Yb3+ ions in the core. Such an effect reduces the total absorption and enhances the upconversion emission, especially the emission from the higher lying level of lanthanide emitters. Moreover, the morphology of the inert shell, which could be controlled during our synthesis, is deemed to be critical to the formation of energy clustering.

Fluorescence Optosensing of Triclosan by Upconversion Nanoparticles with Potassium Permanganate

It is greatly significant to develop a simple and rapid sensing method for triclosan (TCS) because it is a widely used and a chronically toxic compound that adversely affects biological organisms and human health. This paper presents the design and development of a novel simple optosensor that uses carboxylic group-functionalized NaYF₄:Yb³⁺/Er³⁺ upconversion nanoparticles (UCNPs) coated with potassium permanganate (KMnO₄). The sensor enables the rapid, non-autofluorescence, sensitive, and selective detection of TCS based on the "turn off-on fluorescence" technique through fluorescence resonance energy transfer. Under an near-infrared radiation excitation (980 nm), the "turn-off fluorescence" process involves the transfer of fluorescence resonance energy between the UCNPs and KMnO₄, whereas the "turn-on fluorescence" process occurs when KMnO₄ is reduced in the presence of TCS. TCS was detected by recovering the green emission of UCNPs. Under optimized conditions, the resulting sensor offered an excellent response to TCS with 0.2 μM of a limit of detection. The developed sensor showed higher selectivity to TCS than other phenolic compounds. Moreover, the analytical performance of the proposed probe was practically demonstrated to successfully monitor trace levels of TCS in samples of tap water and personal care products. The developed simple and sensitive method may offer a new approach for determining TCS in environmental applications.

In vitro selection of DNA aptamers for the development of chemiluminescence aptasensor for neuron-specific enolase (NSE) detection

Neuron-specific enolase (NSE) is one of the most commonly used serum tumor biomarker in clinical practice for small cell lung cancer screening, early diagnosis, evaluation of therapeutic efficacy and prognosis. In this study, we obtained DNA aptamers with great affinity and selectivity to NSE via subtractive SELEX approach. After 10 rounds, three candidate aptamers were successfully selected and identified. Their affinities were measured by surface plasmon resonance. Apt-5 aptamer with high binding affinity and good specificity were obtained, which had the dissociation constant (K D) values of 12.26 nM. In addition, electrophoretic mobility shift assay (EMSA) experiment also further indicated that the Apt-5 had a highly specific affinity to NSE without binding to HSA. The circular dichroism (CD) analysis revealed that the three aptamers formed stable B-form, stem-loop conformations. The selected aptamers were used to construct a chemiluminescence (CL) aptasensor biosensing platform to detect NSE from actual serum samples. Experimental results confirmed that the CL immunosensing platform had good sensitivity with detection limits of 1-100 ng mL-1. The results demonstrated that our obtained the Apt-5 aptsensor was highly specific in the detection of NSE in serum samples. The detection limit was 0.1 ng mL-1, which was lower than the 0.25 ng mL-1 limit of the ELISA used at the hospital. Moreover, the aptasensor can contribute to better detection of small cell lung cancer (SCLC).

Advances in nanomaterials for use in photothermal and photodynamic therapeutics (Review)

Nanomaterials play crucial roles in the diagnosis and treatment of diseases. Photothermal and photodynamic therapy, as two minimally invasive therapeutic methods, have promising potential in the diagnosis and prevention of cancer. Recently, many photothermal materials (such as noble metal material, transition metal sulfur oxides, carbon material and upconversion nanomaterial) and photodynamic materials (such as phthalein cyanogen, porphyrins and other dye molecules) have been applied in photothermal therapy (PTT) and photodynamic therapy (PDT). Moreover, as nanomaterials have suitable biocompatibility, these materials have been applied in cancer therapy. In the present review, we summarized the effects of different material types, synthesis methods, material morphologies and surface modifications on the outcomes of cancer therapy. The application of nanomaterials in PTT and PDT was introduced and the advantages and disadvantages of PTT and PDT in the prevention of cancer were discussed. Finally, we discussed the application of nanomaterials in the combination of PTT and PDT in cancer treatment.

Upconversion-mediated ZnFe2O4 nanoplatform for NIR-enhanced chemodynamic and photodynamic therapy

ZnFe2O4, a semiconductor catalyst with high photocatalytic activity, is ultrasensitive to ultraviolet (UV) light and tumor H2O2 for producing reactive oxygen species (ROS). Thereby, ZnFe2O4 can be used for photodynamic therapy (PDT) from direct electron transfer and the newly defined chemodynamic therapy (CDT) from the Fenton reaction. However, UV light has confined applicability because of its high phototoxicity, low penetration, and speedy attenuation in the biotissue. Herein, an upconversion-mediated nanoplatform with a mesoporous ZnFe2O4 shell was developed for near-infrared (NIR) light enhanced CDT and PDT. The nanoplatform (denoted as Y-UCSZ) was comprised of upconversion nanoparticles (UCNPs), silica shell, and mesoporous ZnFe2O4 shell and was synthesized through a facile hydrothermal method. The UCNPs can efficiently transfer penetrable NIR photons to UV light, which can activate ZnFe2O4 for producing singlet oxygen thus promoting the Fenton reaction for ROS generation. Besides, Y-UCSZ possesses enormous internal space, which is highly beneficial for housing DOX (doxorubicin, a chemotherapeutic agent) to realize chemotherapy. Moreover, the T 2-weighted magnetic resonance imaging (MRI) effect from Fe3+ and Gd3+ ions in combination with the inherent upconversion luminescence (UCL) imaging and computed tomography (CT) from the UCNPs makes an all-in-one diagnosis and treatment system. Importantly, in vitro and in vivo assays authenticated excellent biocompatibility of the PEGylated Y-UCSZ (PEG/Y-UCSZ) and high anticancer effectiveness of the DOX loaded PEG/Y-UCSZ (PEG/Y-UCSZ&DOX), indicating its potential application in the cancer treatment field.

Selective cellular imaging with lanthanide-based upconversion nanoparticles

Upconversion nanoparticles (UCNPs) with sodium yttrium fluoride, NaYF4 (host lattice) doped with Yb3+ (sensitizer) and Er3+ (activator) were synthesized via hydrothermal route incorporating polyethyleneimine (PEI) for their long-term stability in water. The cationic PEI-modified UCNPs with diameter 20 ± 4 nm showed a zeta potential value of +36.5 mV and showed an intense, visible red luminescence and low-intensity green emission with 976 nm laser excitation. The particles proven to be nontoxic to endothelial cells, with a 3-(4,5-dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide (MTT) assay, showing 90% to 100% cell viability, across a wide range of UCNP concentrations (0.3 ng/mL-0.3 mg/mL) were used in multiphoton imaging. Multiphoton cellular imaging and emission spectroscopy data reported here prove that the UCNPs dispersed in cell culture media are predominantly concentrated in the cytoplasm than the cell nucleus. The energy transfer from PEI-coated UCNPs to surrounding media for red luminescence in the biological system is also highlighted with spectroscopic measurements. Results of this study propose that UCNPs can, therefore, be used for cytoplasm selective imaging together with multiphoton dyes (eg, 4',6-diamidino-2-phenylindole (DAPI)) that are selective to cell nucleus.

Biomedical applications of nanoflares: Targeted intracellular fluorescence probes

Nanoflares are intracellular probes consisting of oligonucleotides immobilized on various nanoparticles that can recognize intracellular nucleic acids or other analytes, thus releasing a fluorescent reporter dye. Single-stranded DNA (ssDNA) complementary to mRNA for a target gene is constructed containing a 3'-thiol for binding to gold nanoparticles. The ssDNA "recognition sequence" is prehybridized to a shorter DNA complement containing a fluorescent dye that is quenched. The functionalized gold nanoparticles are easily taken up into cells. When the ssDNA recognizes its complementary target, the fluorescent dye is released inside the cells. Different intracellular targets can be detected by nanoflares, such as mRNAs coding for genes over-expressed in cancer (epithelial-mesenchymal transition, oncogenes, thymidine kinase, telomerase, etc.), intracellular levels of ATP, pH values and inorganic ions can also be measured. Advantages include high transfection efficiency, enzymatic stability, good optical properties, biocompatibility, high selectivity and specificity. Multiplexed assays and FRET-based systems have been designed.

NIR Biosensing of Neurotransmitters in Stem Cell-Derived Neural Interface Using Advanced Core-Shell Upconversion Nanoparticles

Nondestructive neurotransmitter detection and real-time monitoring of stem cell differentiation are both of great significance in the field of neurodegenerative disease and regenerative medicine. Although luminescent biosensing nanoprobes have been developed to address this need, they have intrinsic limitations such as autofluorescence, scattering, and phototoxicity. Upconversion nanoparticles (UCNPs) have gained increasing attention for various biomedical applications due to their high photostability, low auto-fluorescent background, and deep tissue penetration; however, UCNPs also suffer from low emission intensities due to undesirable energy migration pathways. To address the aforementioned issue, a single-crystal core-shell-shell "sandwich" structured UCNP is developed that is designed to minimize deleterious energy back-transfer to yield bright visible emissions using low power density excitations. These UCNPs show a remarkable enhancement of luminescent output relative to conventional β-NaYF4:Yb,Er codoped UCNPs and β-NaYF4:Yb,Er@NaYF4:Yb "active shell" alike. Moreover, this advanced core-shell-shell UCNP is subsequently used to develop a highly sensitive biosensor for the ultrasensitive detection of dopamine released from stem cell-derived dopaminergic-neurons. Given the challenges of in situ detection of neurotransmitters, the developed NIR-based biosensing of neurotransmitters in stem cell-derived neural interfaces present a unique tool for investigating single-cell mechanisms associated with dopamine, or other neurotransmitters, and their roles in neurological processes.

Classification of Transitions in Upconversion Luminescence of Lanthanides by Two-Dimensional Correlation Analysis

Upconversion luminescence bands from Yb³⁺/Er³⁺ codoped into a matrix such as NaGdF₄ can show a very complex structure on account of multiple intra-f shell transitions occurring in the presence of random crystal fields. We demonstrate that two-dimensional correlation analysis, applied to such time-integrated luminescence spectra measured as a function of excitation power, allows us to gain substantial information about the states involved in transitions, without any additional theoretical input. The detailed correlation analysis allows us not only to identify the location of various transitions but further to club them into groups on the basis of their quantum mechanical origin, and finally subclassify the transitions with each group depending on whether they have a common initial or final state.

Mass production of poly(ethylene glycol) monooleate-modified core-shell structured upconversion nanoparticles for bio-imaging and photodynamic therapy

Developing robust and high-efficient synthesis approaches has significant importance for the expanded applications of upconversion nanoparticles (UCNPs). Here, we report a high-throughput synthesis strategy to fabricate water-dispersible core-shell structured UCNPs. Firstly, we successfully obtain more than 10 grams core UCNPs with high quality from one-pot reaction using liquid rare-earth precursors. Afterwards, different core-shell structured UCNPs are fabricated by successive layer-by-layer strategy to get enhanced fluorescence property. Finally, the hydrophobic UCNPs are modified with poly(ethylene glycol) monooleate (PEG-OA) though a novel physical grinding method. On the basis of mass-production, we use the as-prepared PEG-UCNPs to construct an 808-nm stimuli photodynamic therapy agent, and apply them in cancer therapy and bio-imaging.

Reactive group effects on the photophysical and biological properties of 2-phenyl-1H-phenanthro[9,10-d]imidazole derivatives as fluorescent markers

The presented research focuses on the theoretical design and procedures for preparing protein conjugates with markers. For this purpose a series of phenanthroimidazole (PhI) analogous compounds was designed and investigated by means of first principle methods. Through the judicious choice of cross-linking reagents and the selection of reactive groups, five target fluorescent probes were selected, one of which was previously described using in vitro tests. For the best cognitive purpose and understanding of the nature of the protein conjugation, the studies describe the impact of the reactive group on the solvatochromism, the polarity of the charge transfer of the excited states, the Stokes' shift, ECD spectra and two-photon cross sections. The research is also extended to an analysis of PhI-Concanavalin A biocomplexes and changes in photophysical properties after conjugation. In order to identify valuable alternatives to commercial probes designed for cellular labelling in biological and biomedical imaging, biological properties were described such as ecotoxicity, log P and log BCF, and dye-protein binding was quantified by means of AutoDock and molecular dynamics simulations. The study showed that for phenanthroimidazole derivatives the factor which limits the possibility of their use in medical imaging is the presence of a pyridyl disulfide group, while the introduction of an N-hydroxysuccinimide ester may be used to create stable and valuable fluorescent probes with a wide spectrum for applications in biomedical imaging.

Methylene Blue-Loaded Upconverting Hydrogel Nanocomposite: Potential Material for Near-Infrared Light-Triggered Photodynamic Therapy Application

The property of upconverting nanoparticles to convert the low-energy near-infrared (NIR) light into high-energy visible light has made them a potential candidate for various biomedical applications including photodynamic therapy (PDT). In this work, we show how a surface functionalization approach on the nanoparticle can be used to develop a nanocomposite hydrogel which can be of potential use for the PDT application. The upconverting hydrogel nanocomposite was synthesized by reacting 10-undecenoic acid-capped Yb3+/Er3+-doped NaYF4 nanoparticles with the thermosensitive N-isopropylacrylamide monomer. The formation of hydrogel was completed within 15 min and hydrogel nanocomposites showed strong enhancement in the visible light emission compared to the emission obtained from 10-undecenoic acid-capped Yb3+/Er3+-doped NaYF4 nanoparticles via the upconversion process (under 980 nm laser excitation). The upconverting hydrogel nanocomposites displayed high swelling behavior in water because of their porous nature. The porous structure ensured a higher loading of methylene blue dye (∼78% in 1 h) into the upconverting hydrogel, which was achieved via the swelling diffusion phenomenon. Upon excitation with the NIR light, the visible light emitted from the hydrogel activated the photosensitizer methylene blue which generated reactive oxygen species. Our results were able to show that the methylene blue-loaded composite hydrogel can be a potential platform for the future of NIR-triggered PDT in skin cancer treatment.

Dual Stimuli-Responsive Block Copolymers for Controlled Release Triggered by Upconversion Luminescence or Temperature Variation

An amphiphilic block copolymer (BCP) which contains both photoresponsive and thermoresponsive blocks was synthesized by the atom transfer radical polymerization approach. Meanwhile, a new core/shell type of the upconversion nanoparticle (UCNP) LiYF4:Yb3+ 0.25,Tm3+ 0.01@LiYF4:Yb3+ 0.2 was successfully synthesized. By encapsulating UCNPs inside the micelles of the BCP and incorporating Nile red (NR) into the UCNP@BCP hybrid nanoparticles as a model drug, controlled release of the drug by the dual-stimuli BCP could be studied. After exposing the UCNP-loaded micellar solution to near-infrared (NIR) light, it was found that the UV light pumped from UCNPs could disrupt the polymer micelles and the fluorescence intensity of NR decreased with the increase of the irradiation time of the NIR light. The thermoresponsive study indicated that the fluorescence intensity of NR decreased with the increase of temperature of the micellar solution because of the release of NR into water arising from the contraction of the amphiphilic BCP.

Resonant Plasmon-Enhanced Upconversion in Monolayers of Core-Shell Nanocrystals: Role of Shell Thickness

The upconversion luminescence (UCL) of colloidal lanthanide-doped upconversion nanocrystals (UCNCs) can be improved either by precise encapsulation of the surface by optically inert shells around the core, by an alteration of the nearby environment via metal nanoparticles, or by a combination of both. Considering their potential importance in crystalline silicon photovoltaics, the present study investigates both effects for two-dimensional arrangements of UCNCs. Using excitation light of 1500 nm wavelength, we study the variation in the upconversion luminescence from an Er³⁺-doped NaYF₄ core as a function of the thickness of a NaLuF₄ shell in colloidal solutions as well as in spin-cast-assisted self-assembled monolayers of UCNCs. The observed UCL yields and decay times of Er³⁺ ions of the UCNCs increase with increasing shell thickness in both cases, and nearly no variation in decay times is observed in the transition of the UCNCs from solution to film configurations. The luminescence efficiency of the UCNC monolayers is further enhanced by electron-beam-lithographic-designed Au nanodiscs deposited either on top of or buried within the monolayer. It is observed that the improvement by the nanocrystal shells is greater than that of the Au nanodiscs.

Lipid-Wrapped Upconversion Nanoconstruct/Photosensitizer Complex for Near-Infrared Light-Mediated Photodynamic Therapy

Photodynamic therapy (PDT) is a noninvasive medical technology that has been applied in cancer treatment where it is accessible by direct or endoscope-assisted light irradiation. To lower phototoxicity and increase tissue penetration depth of light, great effort has been focused on developing new sensitizers that can utilize red or near-infrared (NIR) light for the past decades. Lanthanide-doped upconversion nanoparticles (UCNPs) have a unique property to transduce NIR excitation light to UV-vis emission efficiently. This property allows some low-cost, low-toxicity, commercially available visible light sensitizers, which originally are not suitable for deep tissue PDT, to be activated by NIR light and have been reported extensively in the past few years. However, some issues still remain in the UCNP-assisted PDT platform such as colloidal stability, photosensitizer loading efficiency, and accessibility for targeting ligand installation, despite some advances in this direction. In this study, we designed a facile phospholipid-coated UCNP method to generate a highly colloidally stable nanoplatform that can effectively load a series of visible light sensitizers in the lipid layers. The loading stability and singlet oxygen generation efficiency of this sensitizer-loaded lipid-coated UCNP platform were investigated. We also have demonstrated the enhanced cellular uptake efficiency and tumor cell selectivity of this lipid-coated UCNP platform by changing the lipid dopant. On the basis of the evidence of our results, the lipid-complexed UCNP nanoparticles could serve as an effective photosensitizer carrier for NIR light-mediated PDT.

Novel one pot synthesis and spectroscopic characterization of a folate-Mn3O4 nanohybrid for potential photodynamic therapeutic application

Treatment of cancer using nanoparticles made of inorganic and metallic compounds has been increasingly used, owing to their novel intrinsic physical properties and their potential to interact with specific cellular sites, thereby significantly reducing severe secondary effects. In this study, we report a facile strategy for synthesis of folate capped Mn₃O₄ nanoparticles (FA-Mn₃O₄ NPs) with high colloidal stability in aqueous media using a hydrothermal method for potential application in photodynamic therapy (PDT) of cancer. The capping of FA to Mn₃O₄ NPs was confirmed using various spectroscopic techniques. In adenocarcinomic human alveolar basal epithelial cells (A549), the nanohybrid synthesised with a combination of FA and Mn₃O₄ shows remarkable PDT activity via intracellular ROS generation (singlet oxygen). As established by a DNA fragmentation assay and fluorescence studies, the nanohybrid can cause significant nuclear DNA damage by light induced enhanced ROS generation. In the assessment of Bax, Bcl2 provides strong evidence of apoptotic cellular death. Cumulatively, the outcomes of this study suggest that these newly synthesized FA-Mn₃O₄ NPs can specifically destroy cells with overexpressed folate receptors, thereby providing a solution in the journey of cancer eradication.

Folate capped Mn₃O₄ nanoparticles can be used in PDT for specifically destroying folate receptor-overexpressing cancer cells through photo induced free radical damage.

Multifunctional NaYF4:Yb,Er@PE3@Fe3O4 nanocomposites for magnetic-field-assisted upconversion imaging guided photothermal therapy of cancer cells

Magnetic-field-assisted sensitive upconversion luminescence imaging and enhanced photothermal therapeutic efficacy were achieved with multifunctional NaYF₄:Yb,Er@PE3@Fe₃O₄ nanocomposites.

Near-Infrared Manipulation of Membrane Ion Channels via Upconversion Optogenetics

Membrane ion channels are ultimately responsible for the propagation and integration of electrical signals in the nervous, muscular, and other systems. Their activation or malfunctioning plays a significant role in physiological and pathophysiological processes. Using optogenetics to dynamically and spatiotemporally control ion channels has recently attracted considerable attention. However, most of the established optogenetic tools (e.g., channelrhodopsins, ChRs) for optical manipulations, are mainly stimulated by UV or visible light, which raises the concerns of potential photodamage, limited tissue penetration, and high-invasive implantation of optical fiber devices. Near-infrared (NIR) upconversion nanoparticle (UCNP)-mediated optogenetic systems provide great opportunities for overcoming the problems encountered in the manipulation of ion channels in deep tissues. Hence, this review focuses on the recent advances in NIR regulation of membrane ion channels via upconversion optogenetics in biomedical research. The engineering and applications of upconversion optogenetic systems by the incorporation multiple emissive UCNPs into various light-gated ChRs/ligands are first elaborated, followed by a detailed discussion of the technical improvements for more precise and efficient control of membrane channels. Finally, the future perspectives for refining and advancing NIR-mediated upconversion optogenetics into in vivo even in clinical applications are proposed.

Photo-triggered polymer nanomedicines: From molecular mechanisms to therapeutic applications

The use of nanotechnology to improve treatment efficacy and reduce side effects is central to nanomedicine. In this context, stimuli-responsive drug delivery systems (DDS) such as chemical/physical gels or nanoparticles such as polymersomes, micelles or nanogels are particularly promising and are the focus of this review. Several stimuli have been considered but light as an exogenous trigger presents many advantages that are pertinent for clinical applications such as high spatial and temporal control and low cost. Underlying mechanisms required for the release of therapeutic agents in vitro and in vivo range from the molecular scale, namely photoisomerization, hydrophobicity photoswitching, photocleavage or heat generation via nanoheaters, through to the macromolecular scale. As well as these approaches, DDS destabilization, DDS permeation pore unblocking and formation are discussed.

Upconversion luminescence in sub-10 nm β-NaGdF4:Yb3+,Er3+ nanoparticles: an improved synthesis in anhydrous ionic liquids

The microwave-assisted synthesis of β-NaGdF₄:Er³⁺,Yb³⁺ in anhydrous ionic liquids yields efficient upconversion luminescence nanoparticles. A core–shell structure raises the nanoparticle emission intensity to 0.12% of the bulk material.