Emission Of Upconversion Nanoparticles Research Articles

Target-Triggered Self-Assembly of Single-Band Red Emission of Upconversion Nanoparticle-Based Dual-Signal Fluorescent Probes for Sensitive and Accurate Determination of microRNAs

MicroRNAs (miRNAs) are small molecules with significant regulatory functions that have served as biomarkers for the early diagnosis of disease. Accordingly, it is purposeful to develop a rapid, targeted assay for the sensitive, selective, and determination of target miRNAs. Single-band red emission of upconversion nanoparticles (UCNPs) offer high photostability, large anti-Stokes shifts, and low background. Additionally, improving the accuracy via dual-signal fluorescence is also effective for reducing false positive signals induced by background. Herein, taking advantage of the single-band red emission of UCNPs and DNA complementary nanotechnologies, a target-triggered self-assembled dual-signal response biosensor via a strand displacement cascade is reported. The reported approach was used for the determination of miRNA-222 (a model analyte) in serum and the results were similar to those obtained by the polymerase chain reaction (PCR). The spiked recoveries were from 92.5 to 108.4%, indicating that this biosensor has potential for accurate and sensitive determination of miRNAs in biological samples.

Large-Area Near-Infrared Emission Enhancement on Single Upconversion Nanoparticles by Metal Nanohole Array.

Single lanthanide (Ln) ion doped upconversion nanoparticles (UCNPs) exhibit great potential for biomolecule sensing and counting. Plasmonic structures can improve the emission efficiency of single UCNPs by modulating the energy transferring process. Yet, achieving robust and large-area single UCNP emission modulation remains a challenge, which obstructs investigation and application of single UCNPs. Here, we present a strategy using metal nanohole arrays (NHAs) to achieve energy-transfer modulation on single UCNPs simultaneously within large-area plasmonic structures. By coupling surface plasmon polaritons (SPPs) with higher-intermediate state (1D2 → 3F3, 1D2 → 3H4) transitions, we achieved a remarkable up to 10-fold enhancement in 800 nm emission, surpassing the conventional approach of coupling SPPs with an intermediate ground state (3H4 → 3H6). We numerically simulate the electrical field distribution and reveal that luminescent enhancement is robust and insensitive to the exact location of particles. It is anticipated that the strategy provides a platform for widely exploring applications in single-particle quantitative biosensing.

A scalable fabrication method for gold nanodisk-upconverting nanoparticle hybrid nanostructures.

Plasmonic nanostructures can be used to enhance the efficiency of upconversion nanoparticles (UCNPs) and enable new functionalities. However, the fabrication of these hybrid plasmon-UCNP nanostructures has traditionally relied on either wet chemistry or nanolithography routes that are difficult to control, scale up, or both. In this work, we present a scalable nanofabrication process, capable of producing a massive array of gold-UCNP hybrid nanostructures over a few mm2 area and with excellent uniformity in the photoluminescence intensity. This new approach combines the scalability of the bottom-up self-assembly method and the precision of the top-down nanolithography approach. It provides an efficient alternative route for the production of plasmonically enhanced UCNPs. A detailed discussion on the optimization of the UCNP self-assembly, the gold nanodisk lithography, and the nanopattern transfer processes is presented here. Additionally, we showcase the potential of this new approach for fabricating mechanical force sensors based on the selective plasmonic enhancement of the UCNP emission. This new approach holds great potential in facilitating the production of plasmonically enhanced UCNPs that can be deployed for both imaging and sensing applications.

Indocyanine green-equipped upconversion nanoparticles/CeO2 trigger mutually reinforced dual photodynamic therapy

Although photodynamic-based tumor therapy has attracted considerable attention, the antitumor effect is limited due to the low reactive oxygen species (ROS) generation of photosensitizers. Therefore, it is an urgent need for improving ROS production and promoting therapeutic outcome of tumor. We developed a nanoplatform of indocyanine green (ICG)-equipped upconversion nanoparticles (UCNPs)/CeO2 modified with NH2-PEG1000-cRGDfK (UICCIP). This nanoplatform has two pathways of ROS production for mutually enhanced dual photodynamic therapy (PDT) of tumors. The ultraviolet (UV) emission of UCNPs could trigger CeO2 to produce ROS for PDT under 808 nm laser irradiation, and ICG can also produce singlet oxygen (1O2) at the meantime. Importantly, ICG can enhance the luminescence of UCNPs through energy transfer, thus promoting the PDT effect of CeO2. For another, CeO2 could catalyze endogenous H2O2 to provide O2 then overcome the hypoxic environment of tumor to enhance the PDT effect. By highlighting the full use of the various components of the nanoplatform and establishing interrelationships, ROS production can be increased significantly, resulting in efficient PDT of tumor. It is believed that UICCIP will become a potential ROS producer, which will broaden the thought of exploring new antitumor nanoplatforms.

UV Emission from Lanthanide-Doped Upconversion Nanoparticles in Super-Resolution Microscopy: Potential for Cellular Damage

Upconversion nanoparticles (UCNPs) co-doped with lanthanide ions have recently attracted significant attention as fluorescent probes for super-resolution microscopy (SRM). This is due to the advantages of UCNPs over other fluorescence probes, such as fluorescent proteins, owing to their unique optical properties, limited photobleaching, and sharp emissions. However, the concurrent emission of ultraviolet (UV) wavelength radiation by UCNPs and the potential for cell photodamage, which may limit useful live-cell analysis, have been overlooked. Here, UCNPs synthesized with eight commonly used combinations of Yb/Tm and Yb/Tm/Gd dopants were excited by either pulsed- or continuous-wave (CW) lasers to evaluate their UV emission. The ratio of emitted UV-A and UV-B was measured relative to blue emission at 475 nm, which is traditionally used for imaging during SRM. We demonstrate that most UCNP samples emit UV light and that the dopant concentration has a key role in generating UV emissions. In addition, the use of pulsed or CW lasers for excitation can lead to a large variation in the amount of UV emitted. This work highlights the importance of considering upconversion dopant composition and concentration, as well as analyzing the emission of synthesized UCNPs before their use to prevent unwanted cell photodamage during live-cell imaging by SRM. Moreover, it established a need to improve the visible light emission of UCNPs with respect to UV emission for SRM applications.

Colorimetric and Fluorescent Determination of Alkaline Phosphatase and Ascorbic Acid based upon the Inner Filter Effect using up-Conversion Nanoparticles

A novel fluorescence and colorimetric dual probe based upon the inner filter effect (IFE) is reported to selectively and sensitively determine alkaline phosphatase (ALP). First, up-conversion nanoparticles (UCNPs) were prepared via a facile hydrothermal method and characterized by transmission electron microscopy (TEM), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), and zeta potential. Under near infrared (NIR) excitation at 980 nm, the prepared nanoparticles emit green light at 541 nm. The absorption band of p-phenylenediamine (PPD) overlaps the emission of UCNPs, which effectively quenches the fluorescence of UCNPs. However, ascorbic acid (AA) generated from the reaction of ascorbic acid 2-phosphate (AAP) and ALP impedes the oxidation of PPD and the fluorescence is restored. Based on this change, a “turn-on” sensor was used to determine AA and ALP with the limits of detection of 2.49 μM and 0.018 U/L, respectively. The constructed assay was applied to determine ALP in serum and the recoveries were from 93.3% to 101.3% with a relative standard deviation from 1.1% to 3.3%. A facile and efficient approach is reported for the determination ALP and offers a theoretical basis for the related diseases.

Near-infrared light triggered in situ release of CO for enhanced therapy of glioblastoma

BackgroundPhotodynamic therapy (PDT) features high biocompatibility and high spatiotemporal selectivity, showing a great potential in glioblastoma (GBM) treatment. However, its application was restricted by the poor therapeutic efficacy and side effect.ResultsIn this study, a therapeutic nanoplatform (UCNPs@Ce6/3HBQ@CM) with combination of PDT and CO therapy was constructed, in which a photoCORM and a photosensitizer were loaded onto the surface of upconversion nanoparticles (UCNPs) functioning as photon transducer. Benefitting from NIR excitation and multicolor emission of UCNPs, the penetration depth of excitation light is enhanced and meanwhile simultaneous generation of CO and ROS in tumor site can be achieved. The as-prepared nanocomposite possessed an elevated therapeutic efficiency with the assistance of CO through influencing mitochondrial respiration and depleting ATP, accompanying with the reduced inflammatory responses. By wrapping a homologous cell membrane, the nanocomposite can target GBM and accumulate in the tumor site, affording a powerful tool for precise and efficient treatment of GBM.ConclusionThis therapeutic nanoplatform UCNPs@Ce6/3HBQ@CM, which combines PDT and CO therapy enables precise and efficient treatment of refractory glioblastoma.

Engineering a polymer-encapsulated manganese dioxide/upconversion nanoprobe for FRET-based hydrogen peroxide detection.

Hydrogen peroxide (H2O2) is considered a significant biomarker in various diseases and could induce deleterious health problems at irregular physiological concentrations. Therefore, developing a simple, efficient biocompatible nanoprobe for trace amount H2O2 detection with high sensitivity and specificity is of great help for early diagnosis and therapeutics. Herein, we designed amphiphilic poly(styrene-co-maleic anhydride) (PMSA)-encapsulated nanoclusters composed of upconversion nanoparticles (UCNPs) and manganese dioxide nanoparticles (MnO2 NPs) at a specific ratio to produce a near-infrared (NIR) excited luminescent nanoprobe for H2O2 detection. Our results revealed that the MnO2 NPs tended to experience catalytic decomposition when exposed to H2O2, while the UCNPs were retained inside the PSMA encapsulation, causing recovery of the UCNP emission band at 470nm in accordance with H2O2 concentration. This luminescence recovery was linearly dependent on H2O2 concentrations, yielding a limit of detection (LOD) of 20nM. The easy-to-interpret H2O2 nanoprobe also proved high selectivity in the presence of other interfering substances, and biocompatibility and water-dispersibility, making it an ideal candidate for real-time detection of disease-related H2O2 in living organisms.

Efficient ROS activation by highly stabilized aqueous ICG encapsulated upconversion nanoparticles for tumor cell imaging and therapeutics

Dye-sensitized upconversion nanoparticles (UCNPs) have gained significant interest in biological applications. However, previous attempts to utilize these dye-sensitized UCNPs in the aqueous environment suffer from dramatic decline in the upconversion luminescence (UCL) intensity and problems with nanoparticles aggregation and water quenching. By employing indocyanine green (ICG) as a hydrophobic amplifier and energy harvester for UCNPs, we use a general, facile, and kinetically controlled mixing technology—flash nanoprecipitation (FNP)—to prepare aqueous stable ICG-sensitized UCNPs encapsulated by a biocompatible block copolymer poly (ethylene glycol)-b-poly (lactic-co-glycolic acid (PEG-b-PLGA) to efficiently activate ROS generation for simultaneous cancer cell imaging and treatment. As compared to NPs obtained from traditional drip and stir method (TM), the NPs under high turbulent conditions (high Re) exhibit smaller size, narrower size distribution, and much higher stability, for the UCNPs hydrophobicity enhanced by ICG which was bound on the surface of UCNPs due to the coordination effect between sulfonates of ICG molecules and naked lanthanide metal ions of UCNPs. The remained UCL intensity was significantly boosted as well as ROS generation through FNP in the presence of ICG, but not for TM samples indicating superior protection of upconversion NPs from water quenching and the energy transfer between ICG and UCNP was well retained. The efficiency of ROS generation of NPs via FNP evidently increases with the stronger red-light emission of UCNPs, resulting in high cytotoxicity for SMMC-7721 cells in vitro and good suppression of tumor growth in vivo. The success of NPs by FNP in biological application demonstrates a bright prospect of FNP for synthesis of efficient antitumor nanoparticles.

Dual‐Functional Nonmetallic Plasmonic Hybrids with Three‐Order Enhanced Upconversion Emission and Photothermal Bio‐Therapy

AbstractPlasmonic semiconductor nanoparticles (NPs) with wide‐range tailorable localized surface plasmon resonance (LSPR) hold exciting prospects on optical signal amplification. In this work, by precisely controlling oxygen vacancies around W atoms, plasmonic bismuth tungstate Bi2WO6 (BWO) nanosheets are constructed to enhance emission of Yb3+/Er3+ co‐doped NaYF4 upconversion nanoparticles (UCNPs). In the optimal conditions, the UCNPs/BWO‐2 hybrids exhibit over three‐order (1260‐fold) enhancement selectively on the 520 nm emission owing to the strong LSPR‐induced electrical field and photothermal effect. Moreover, it is found that the highly efficient emission of UCNPs/BWO‐2 allows it to act as a thermometer to monitor the real‐time local temperature with high absolute sensitivity of 5.8 × 10−3 K−1 in wide temperature range (up to 990 K). For proof‐of‐concept, the dual functions of plasmonic UCNPs/BWO‐2 hybrids on bioimaging and photothermal therapy for cancer cells are demonstrated that can be completely killed within 5 min under 980 nm irradiation. As far as it is known, this work reaches a new level on UCNPs emission enhancement by plasmonic semiconductor, exceeding most plasmonic metals.

DNA nanomachine activation and Zn2+ imaging in living cells with single NIR irradiation

Though activatable DNA nanomachines in response to external and internal stimuli successfully improve the temporal and spatial controllability of biosensing and bioimaging, current strategies usually apply different irradiation lights for DNA nanomachine activation and imaging processes. The involvement of short wavelength excitation results in shallow tissue penetration and high background interference. Here we design a photo-locked DNA nanomachine (P-DNA nanomachine) that uses single NIR irradiation for spatiotemporally activating nanomachine operation and metal ion imaging in cancer cells. Upconversion nanoparticles (UCNPs) are modified with energy collector dye FITC, photo-locked DNAzyme and its substrate strands labelled with BHQ1. Part of the UCNPs emission at 450 nm was collected by FITC and quenched by BHQ1 originally. 980 nm irradiation photolyzes PC linker, activates DNAzyme catalytic reaction in the presence of Zn2+, and recovers FITC luminance at 540 nm. The intracellular Zn2+ amount was imaged by quantitatively measuring FITC recovery intensity versus internal standard of UCNPs luminance at 450 nm. The presented NIR activatable P-DNA nanomachine demonstrates effective protection against “false positive” signal from extracellular interference, thus has potential application for in vivo imaging.

Upconversion photoluminescent Yb(Ⅲ)/Er(Ⅲ) doped nanoparticles interact with AuNPs for detection of Cr (Ⅲ) and sodium tripolyphosphate based on FRET

In this study, we designed a dual functional nanosensor for detecting Cr (Ⅲ) and sodium tripolyphosphate (STPP) based on fluorescence resonance energy transfer (FRET) mechanism between amine functionalized upconversion nanoparticles (UCNPs) and gold nanoparticles (AuNPs), where UCNPs serve as energy donor and aggregated AuNPs as energy acceptor. UCNPs have red emission at 654 nm under 980 nm laser excitation and monodispersed AuNPs show strong absorbance around 520 nm. Monodispersed AuNPs form aggregate easily in the presence of Cr (Ⅲ), which leads to the red-shift of the absorption of AuNPs and quenches the red emission of UCNPs. In the presence of STPP, the formation of Cr (Ⅲ)-STPP complex weakens the aggregation extent of AuNPs and the red emission of UCNPs increases. Therefore, the content of Cr (Ⅲ) and STPP could be detected in wide range according to the linear variety of fluorescence intensities of UCNPs at 654 nm with the detection limits of 0.44 μM and 0.54 μM for detecting Cr (Ⅲ) and STPP, respectively. In addition, due to the high selectivity and accuracy for the detection of STPP, this nanosensor can be utilized to determine STPP in green tea drink and drinking water.

Theranostic nanomotors for tumor multimode imaging and photothermal/photodynamic synergistic therapy

Efficient nanotheranostic reagents for cancer has attracted extensive attention. Herein we report nanomotors UCNPs@mSiO2-Au-Cys for tumor multimode imaging and photothermal (PTT)/photodynamic (PDT) combined therapy. Firstly, the photothermal agent Au NPs and photosensitizer Cy-S-Ph-NH2 (Cys) are loaded by upconversion nanoparticles (UCNPs) with the mesoporous silica shell. Nanomotors generate autonomous motion under 808 nm laser irradiation, which can significantly increase their tumor tissue permeability. Meanwhile, Au NPs provide photothermal conversion, photothermal imaging, and act as nanocatalytic enzyme that can enhance the efficacy of oxygen dependent PDT. Cys also can increase the photothermal effect. In addition, the two emission of UCNPs at 659 and 800 nm under 980 nm excitation are offered respectively the possibility to realize real-time online monitoring of therapeutic effect and the tumor fluorescence imaging. This nano-therapeutics platform based on the optical properties of UCNPs and nano-motor technology will provide potential methods for tumor treatment.

ZIF-8 encapsulated upconversion nanoprobes to evaluate pH variations in food spoilage.

Anovel nanoassembly was constructed through encapsulating upconversion nanoparticles (UCNPs) into a metal-organic framework structure (ZIF-8), in which doxorubicin (DOX) was absorbed into pores of ZIF-8. The blue emission of UCNPs was quenched by DOX through the fluorescence resonance energy transfer (FRET) strategy. When the nanoprobe was exposed to food samples with different pH values, ZIF-8 collapsed to release DOX molecules, resulting in upconversion recovery. The porous structure of ZIF-8 provides abundant space for DOX absorption, which significantly improves the detection capacities and accuracy. It is shown that the probe has a good linear relationship when pH values vary from 2.5 to 7.4, and can distinguish pH variations as low as 0.5 in real samples. This strategy has been successfully used to determine food spoilage by determination of pH variations.

A hybrid upconversion nanoprobe for ratiometric detection of aliphatic biogenic amines in aqueous medium.

We fabricated an inorganic–organic hybrid upconversion nanoprobe for the ratiometric detection of aliphatic biogenic amines in water. The hybrid nanoprobe comprises a thiophene-based acceptor–π-donor–π-acceptor organic fluorescent dye, TDPM, and near-infrared light-absorbing upconversion nanoparticles (UCNPs). The organic dye was loaded into a mesoporous silica-coated UCNP (UCNP@mSiO2) matrix to circumvent the issues of water insolubility and higher energy excitation. Yb3+ and Tm3+-doped UCNPs exhibited dual emission bands at 475 and 645 nm upon excitation with a 980 nm laser. The significant spectral overlap between the absorption and the emission bands of TDPM and UCNPs, respectively, at 475 nm led to resonance energy transfer (RET) from the UCNPs to TDPM resulting in the quenching of the UCNP emission. In contrast, ‘turn-on’ emission was noticeable with the addition of aliphatic biogenic amines due to an inhibition of the RET. The emission at 645 nm remained unaffected during the energy transfer process making the hybrid probe a versatile platform for the ratiometric detection of different aliphatic biogenic amines. Furthermore, we explored the sensing of aliphatic biogenic amines in adulterated milk and rotten fish. The unique material attributes demonstrated in the current study hold promise for further development of real-time sensors and switches based on hybrid upconversion nanoprobes.

Core-satellite metal-organic framework@upconversion nanoparticle superstructures via electrostatic self-assembly for efficient photodynamic theranostics

The nanoplatforms based on upconversion nanoparticles (UCNPs) have shown great promise in amplified photodynamic therapy (PDT) triggered by near-infrared (NIR) light. However, their practical in vivo applications are hindered by the overheating effect of 980 nm excitation and low utilization of upconversion luminescence (UCL) by photosensitizers. To solve these defects, core-satellite metal-organic framework@UCNP superstructures, composed of a single metal-organic framework (MOF) NP as the core and Nd3+-sensitized UCNPs as the satellites, are designed and synthesized via a facile electrostatic self-assembly strategy. The superstructures realize a high co-loading capacity of chlorin e6 (Ce6) and rose bengal (RB) benefitted from the highly porous nature of MOF NPs, showing a strong spectral overlap between maximum absorption of photosensitizers and emission of UCNPs. The in vitro and in vivo experiments demonstrate that the dual-photosensitizer superstructures have trimodal (magnetic resonance (MR)/UCL/fluorescence (FL)) imaging functions and excellent antitumor effectiveness of PDT at 808 nm NIR light excitation, avoiding the laser irradiation-induced overheating issue. This study provides new insights for the development of highly efficient PDT nanodrugs toward precision theranostics.

Upconversion nanoparticles as intracellular pH messengers

Upconversion nanoparticles (UCNPs) should be particularly well suited for measurement inside cells because they can be imaged down to submicrometer dimensions in near real time using fluorescence microscopy, and they overcome problems, such as photobleaching, autofluorescence, and deep tissue penetration, that are commonly encountered in cellular imaging applications. In this study, the performance of an UCNP modified with a pH-sensitive dye (pHAb) is studied. The dye (emission wavelength 580 nm) was attached in a polyethylene imine (PEI) coating on the UCNP and excited via the 540-nm UCNP emission under 980-nm excitation. The UC resonance energy transfer efficiencies at different pHs ranged from 25 to 30% and a Förster distance of 2.56 nm was predicted from these results. Human neuroblastoma SH-SY5Y cells, equilibrated with nigericin H+/K+ ionophore to equalize the intra- and extracellular pH‚ showed uptake of the UCNP-pHAb conjugate particles and, taking the ratio of the intensity collected from the pHAb emission channel (565–630 nm) to that from the UCNP red emission channel (640–680 nm), produced a sigmoidal pH response curve with an apparent pKa for the UCNP-pHAb of ~ 5.1. The UCNP-pHAb were shown to colocalize with LysoBrite dye, a lysosome marker. Drug inhibitors such as chlorpromazine (CPZ) and nystatin (NYS) that interfere with clathrin-mediated endocytosis and caveolae-mediated endocytosis, respectively, were investigated to elucidate the mechanism of nanoparticle uptake into the cell. This preliminary study suggests that pH indicator–modified UCNPs such as UCNP-pHAb can report pH in SH-SY5Y cells and that the incorporation of the nanoparticles into the cell occurs via clathrin-mediated endocytosis.Graphical abstract

Electric-Field-Regulated Energy Transfer in Chiral Liquid Crystals for Enhancing Upconverted Circularly Polarized Luminescence through Steering the Photonic Bandgap.

Circularly polarized luminescent materials with high dissymmetry factor (glum ) have been attracting increasing attention due to their distinctive photonic properties. In this work, by incorporating upconversion nanoparticles (UCNPs) and CsPbBr3 perovskite nanocrystals (PKNCs) into a chiral nematic liquid crystal (N*LC), enhanced upconverted circularly polarized luminescence (UC-CPL) based on a radiative energy transfer (RET) process from UCNPs to CsPbBr3 PKNCs is successfully implemented. By locating the emission peak of CsPbBr3 PKNCs at the center of the photonic bandgap of N*LC, the maximum glum value of UC-CPL can be amplified to an extremely large value of 1.1. Meanwhile, upconverted emission of UCNPs can be significantly enhanced due to the band edge enhancement effect of the N*LC, subsequently enhancing the emission of the CsPbBr3 PKNCs through the RET process. In addition, an applied electric field can switch the upconverted emission of the UCNPs, as well as the RET process, enabling an electric-field-controlled UC-CPL switch.

Activatable Photodynamic Therapy with Therapeutic Effect Prediction Based on a Self-correction Upconversion Nanoprobe.

Though emerging as a promising therapeutic approach for cancers, the crucial challenge for photodynamic therapy (PDT) is activatable phototoxicity for selective cancer cell destruction with low "off-target" damage and simultaneous therapeutic effect prediction. Here, we design an upconversion nanoprobe for intracellular cathepsin B (CaB)-responsive PDT with in situ self-corrected therapeutic effect prediction. The upconversion nanoprobe is composed of multishelled upconversion nanoparticles (UCNPs) NaYF4:Gd@NaYF4:Er,Yb@NaYF4:Nd,Yb, which covalently modified with an antenna molecule 800CW for UCNPs luminance enhancement under NIR irradiation, photosensitizer Rose Bengal (RB) for PDT, Cy3 for therapeutic effect prediction, and CaB substrate peptide labeled with a QSY7 quencher. The energy of UCNPs emission at 540 nm is transferred to Cy3/RB and eventually quenched by QSY7 via two continuous luminance resonance energy transfer processes from interior UCNPs to its surface-extended QSY7. The intracellular CaB specifically cleaves peptide to release QSY7, which correspondingly activates RB with reactive oxygen species (ROS) generation for PDT and recovers Cy3 luminance for CaB imaging. UCNPs emission at 540 nm remains unchanged during the peptide cleavage process, which is served as an internal standard for Cy3 luminance correction, and the fluorescence intensity ratio of Cy3 over UCNPs (FI583/FI540) is measured for self-corrected therapeutic effect prediction. The proposed self-corrected upconversion nanoprobe implies significant potential in precise tumor therapy.

Observation of high efficient photothermal conversion of sub-10 nm Au nanoparticles coated on upconversion nanoparticles

Gold nanoparticles have been confirmed to be very suited for applications in cancer photothermal therapy. However, nanoscale photothermal conversion mechanisms of Au nanoparticles are still not completely unveiled. Here, we employ upconversion nanoparticles (UCNPs) as nanoscale light sources, of which green emission centered at 540 nm can be efficiently absorbed by Au nanoparticles with an average diameter of 4 nm. Gold nanoparitcles (~4 nm) are coated on upconversion nanoparticles (UCNPs) with an average diameter of 90 nm, while the gap among these coated Au nanoparticles is smaller than 20 nm which is obviously shorter than the half wavelength (270 nm) of green emission of UCNP core nanoparticles. As a result, green emission of UCNPs could not penetrate the coating of Au nanoparticles without being absorbed. Based on UCNPs@Au nanosystems, high efficient photothermal conversion of Au nanoparticles can be directly observed, while a photothermal conversion efficiency of 41.2% is determined. Importantly, fluorescent resonance energy transfer (FRET) mechanisms from UCNPs to Au nanoparticles is confirmed to be the predominant route for this nanoscale photothermal system (UCNPs@Au), instead of direct photon absorption.