Tuning Tetrazole Photochemistry for Protein Ligation and Molecular Imaging

Photochemistry provides a wide range of alternative reagents that hold potential for use in bimolecular functionalisation of proteins. Here, we report the synthesis and characterisation of metal ion binding chelates derivatised with disubstituted tetrazoles for the photoradiochemical labelling of monoclonal antibodies (mAbs). The photophysical properties of tetrazoles featuring extended aromatic systems and auxochromic substituents to tune excitation toward longer wavelengths (365 and 395 nm) were studied. Two photoactivatable chelates based on desferrioxamine B (DFO) and the aza-macrocycle NODAGA were functionalised with a tetrazole and developed for protein labelling with ⁸⁹Zr, ⁶⁴Cu and ⁶⁸Ga radionuclides. DFO-tetrazole ( 1 ) was assessed by direct conjugation to formulated trastuzumab and subsequent radiolabelling with ⁸⁹Zr. Radiochemical studies and cellular-based binding assays demonstrated that the radiotracer remained stable in vitro retained high immunoreactivity. Positron emission tomography (PET) imaging and biodistribution studies were used to measure the tumour specific uptake and pharmacokinetic profile in mice bearing SK-OV-3 xenografts. Experiments demonstrate that tetrazole-based photochemistry is a viable approach for the light-induced synthesis of PET radiotracers.     

[89Zr]-Pertuzumab PET Imaging Reveals Paclitaxel Treatment Efficacy Is Positively Correlated with HER2 Expression in Human Breast Cancer Xenograft Mouse Models

Paclitaxel (PTX) treatment efficacy varies in breast cancer, yet the underlying mechanism for variable response remains unclear. This study evaluates whether human epidermal growth factor receptor 2 (HER2) expression level utilizing advanced molecular positron emission tomography (PET) imaging is correlated with PTX treatment efficacy in preclinical mouse models of HER2+ breast cancer. HER2 positive (BT474, MDA-MB-361), or HER2 negative (MDA-MB-231) breast cancer cells were subcutaneously injected into athymic nude mice and PTX (15 mg/kg) was administrated. In vivo HER2 expression was quantified through [⁸⁹Zr]-pertuzumab PET/CT imaging. PTX treatment response was quantified by [¹⁸F]-fluorodeoxyglucose ([¹⁸F]-FDG) PET/CT imaging. Spearman’s correlation, Kendall’s tau, Kolmogorov–Smirnov test, and ANOVA were used for statistical analysis. [⁸⁹Zr]-pertuzumab mean standard uptake values (SUV_mean) of BT474 tumors were 4.9 ± 1.5, MDA-MB-361 tumors were 1.4 ± 0.2, and MDA-MB-231 (HER2−) tumors were 1.1 ± 0.4. [¹⁸F]-FDG SUV_mean changes were negatively correlated with [⁸⁹Zr]-pertuzumab SUV_mean (r = −0.5887, p = 0.0030). The baseline [¹⁸F]-FDG SUV_mean was negatively correlated with initial [⁸⁹Zr]-pertuzumab SUV_mean (r = −0.6852, p = 0.0002). This study shows PTX treatment efficacy is positively correlated with HER2 expression level in human breast cancer mouse models. Molecular imaging provides a non-invasive approach to quantify biological interactions, which will help in identifying chemotherapy responders and potentially enhance clinical decision-making.     

Chelate-free metal ion binding and heat-induced radiolabeling of iron oxide nanoparticles

A novel reaction for chelate-free, heat-induced metal ion binding and radiolabeling of ultra-small paramagnetic iron oxide nanoparticles (USPIOs) has been established. Radiochemical and non-radioactive labeling studies demonstrated that the reaction has a wide chemical scope and is applicable to p-, d- and f-block metal ions with varying ionic sizes and formal oxidation states from 2+ to 4+. Radiolabeling studies found that ⁸⁹Zr–Feraheme (⁸⁹Zr–FH or ⁸⁹Zr–ferumoxytol) can be isolated in 93 ± 3% radiochemical yield (RCY) and >98% radiochemical purity using size-exclusion chromatography. ⁸⁹Zr–FH was found to be thermodynamically and kinetically stable in vitro using a series of ligand challenge and plasma stability tests, and in vivo using PET/CT imaging and biodistribution studies in mice. Remarkably, ICP-MS and radiochemistry experiments showed that the same reaction conditions used to produce ⁸⁹Zr–FH can be employed with different radionuclides to yield ⁶⁴Cu–FH (66 ± 6% RCY) and ¹¹¹In–FH (91 ± 2% RCY). Electron magnetic resonance studies support a mechanism of binding involving metal ion association with the surface of the magnetite crystal core. Collectively, these data suggest that chelate-free labeling methods can be employed to facilitate clinical translation of a new class of multimodality PET/MRI radiotracers derived from metal-based nanoparticles. Further, this discovery is likely to have broader implications in drug delivery, metal separation science, ecotoxicology of nanoparticles and beyond.     

A Semi Rigid Novel Hydroxamate AMPED-Based Ligand for 89Zr PET Imaging

In this work, we designed, developed, characterized, and investigated a new chelator and its bifunctional derivative for ⁸⁹Zr labeling and PET-imaging. In a preliminary study, we synthesized two hexadentate chelators named AAZTHAS and AAZTHAG, based on the seven-membered heterocycle AMPED (6-amino-6-methylperhydro-1,4-diazepine) with the aim to increase the rigidity of the ⁸⁹Zr complex by using N-methyl-N-(hydroxy)succinamide or N-methyl-N-(hydroxy)glutaramide pendant arms attached to the cyclic structure. N-methylhydroxamate groups are the donor groups chosen to efficiently coordinate ⁸⁹Zr. After in vitro stability tests, we selected the chelator with longer arms, AAZTHAG, as the best complexing agent for ⁸⁹Zr presenting a stability of 86.4 ± 5.5% in human serum (HS) for at least 72 h. Small animal PET/CT static scans acquired at different time points (up to 24 h) and ex vivo organ distribution studies were then carried out in healthy nude mice (n = 3) to investigate the stability and biodistribution in vivo of this new ⁸⁹Zr-based complex. High stability in vivo, with low accumulation of free ⁸⁹Zr in bones and kidneys, was measured. Furthermore, an activated ester functionalized version of AAZTHAG was synthesized to allow the conjugation with biomolecules such as antibodies. The bifunctional chelator was then conjugated to the human anti-HER2 monoclonal antibody Trastuzumab (Tz) as a proof of principle test of conjugation to biologically active molecules. The final ⁸⁹Zr labeled compound was characterized via radio-HPLC and SDS-PAGE followed by autoradiography, and its stability in different solutions was assessed for at least 4 days.     

Rational Design,Synthesis, and Evaluation of Tetrahydroxamic Acid Chelators for Stable Complexation of Zirconium(IV)

Metals of interest for biomedical applications often need to be complexed and associated in a stable manner with a targeting agent before use. Whereas the fundamentals of most transition‐metal complexation processes have been thoroughly studied, the complexation of Zr^IV has been somewhat neglected. This metal has received growing attention in recent years, especially in nuclear medicine, with the use of ⁸⁹Zr, which a β⁺‐emitter with near ideal characteristics for cancer imaging. However, the best chelating agent known for this radionuclide is the trishydroxamate desferrioxamine B (DFB), the Zr^IV complex of which exhibits suboptimal stability, resulting in the progressive release of ⁸⁹Zr in vivo. Based on a recent report demonstrating the higher thermodynamic stability of the tetrahydroxamate complexes of Zr^IV compared with the trishydroxamate complexes analogues to DFB, we designed a series of tetrahydroxamic acids of varying geometries for improved complexation of this metal. Three macrocycles differing in their cavity size (28 to 36‐membered rings) were synthesized by using a ring‐closing metathesis strategy, as well as their acyclic analogues. A solution study with ⁸⁹Zr showed the complexation to be more effective with increasing cavity size. Evaluation of the kinetic inertness of these new complexes in ethylenediaminetetraacetic acid (EDTA) solution showed significantly improved stabilities of the larger chelates compared with ⁸⁹Zr‐DFB, whereas the smaller complexes suffered from insufficient stabilities. These results were rationalized by a quantum chemical study. The lower stability of the smaller chelates was attributed to ring strain, whereas the better stability of the larger cyclic complexes was explained by the macrocyclic effect and by the structural rigidity. Overall, these new chelating agents open new perspectives for the safe and efficient use of ⁸⁹Zr in nuclear imaging, with the best chelators providing dramatically improved stabilities compared with the reference DFB.     

Screening for a 177Lu-labeled CA19–9 monoclonal antibody via PET imaging for colorectal cancer therapy

Carbohydrate antigen 19–9 (CA19–9) with multi epitopes relatively high expresses on colorectal cancer (CRC) cells, making it an attractive target for developing radioimmunotherapy (RIT) for CRC. The lutetium-177 (¹⁷⁷Lu) labeled monoclonal antibodies (mAbs) can selectively bind the corresponding antigens and release targeted cytotoxic radiation, which could induce cell apoptosis and reduce the drug-induced resistance. Here, a series of CA19–9 mAbs were labeled with zirconium-89 (⁸⁹Zr), and one with high tumor uptake was screened via PET imaging, which has potential application for the diagnosis of CRC. Then the screened mAb (C003) labeled with ¹⁷⁷Lu was utilized for CA19–9 targeted RIT, which presents a significant suppression effect on the growth of colo205 xenografts than immunotherapy alone. Meanwhile, the side effects of ¹⁷⁷Lu-DOTA-C003 are limited according to the results of in vivo study. Both ⁸⁹Zr-DFO-C003 for CRC immune-PET imaging and ¹⁷⁷Lu-DOTA-C003 for RIT against CRC exhibit good potential in clinical applications.     

Synthesis and Feasibility Evaluation of a new Trastuzumab Conjugate Integrated with Paclitaxel and 89Zr for Theranostic Application Against HER2-Expressing Breast Cancers

The preparation and in vitro evaluation of a theranostic conjugate composed of trastuzumab, paclitaxel (PTX), and deferoxamine (DFO)-chelated ⁸⁹Zr have been reported. These comounds have potential applications against HER2 receptor positive breast cancers. We conjugated DFO and PTX to trastuzumab by exploiting simple conjugation chemistry. The conjugate (DFO-trastuzumab-PTX) showed excellent radiolabeling efficiency with ⁸⁹Zr and the labeled conjugate had high in vitro stability in human serum. Furthermore, DFO-trastuzumab-PTX displayed comparable cytotoxicity with PTX and ⁸⁹Zr-DFO-trastuzumab-PTX exhibited HER2 receptor-mediated binding on HER2-positive MDA-MB-231 breast cancer cells. The results of our in vitro study indicate high potential of ⁸⁹Zr-DFO-trastuzumab-PTX to be utilized in the theranostic application against HER2-postive breast cancers.     

Ag@TiO2 Nanoprisms with Highly Efficient Near‐Infrared Photothermal Conversion for Melanoma Therapy

Melanoma is a primary reason of death from skin cancer and associated with high lethality. Photothermal therapy (PTT) has been developed into a powerful cancer treatment technique in recent years. Here, we created a low‐cost and high‐performance PTT agent, Ag@TiO₂ NPs, which possesses a high photothermal conversion efficiency of ≈65 % and strong near‐infrared (NIR) absorption about 808 nm. Ag NPs were synthesized using a two‐step method and coated with TiO₂ to obtain Ag@TiO₂ NPs by a facile sol‐gel method. Because of the oxide, Ag@TiO₂ NPs exhibit remarkable high photothermal conversion efficiencies and biocompatibility in vivo and in vitro. Cytotoxicity and therapeutic efficiency of photothermal cytotoxicity of Ag@TiO₂ NPs were tested in B16‐F10 cells and C57BL/6J mice. Under light irradiation, the elevated temperature causes cell death in Ag NPs‐treated (100 μg mL^?1) cells in vitro (both p<0.01). In the case of subcutaneous melanoma tumor model, Ag@TiO₂ NPs (100 μg mL^?1) were injected into the tumor and irradiated with a 808 nm laser of 2 W cm^?2 for 1 minute. As a consequence, the tumor volume gradually decreased by NIR laser irradiation with only a single treatment. The results demonstrate that Ag@TiO₂ NPs are biocompatible and an attractive photothermal agent for cutaneous melanoma by local delivery.     

Photochemical Conjugation and One‐Pot Radiolabelling of Antibodies for Immuno‐PET

Monoclonal antibodies (mAbs), immunoglobulin fragments, and other proteins are important scaffolds in the development of radiopharmaceuticals for diagnostic immuno‐positron emission tomography (immuno‐PET) and targeted radioimmunotherapy (RIT). Conventional methods for radiolabelling proteins with metal ions such as ⁶⁸Ga, ⁶⁴Cu, ⁸⁹Zr, and ⁹⁰Y require multi‐step procedures involving pre‐purification, functionalisation with a chelate, and subsequent radiolabelling. Standard coupling chemistries are time‐consuming, difficult to automate, and involve synthesis, isolation, and storage of an intermediate, new molecular entity (the conjugated mAb) whose biochemical properties can differ from those of the parent protein. To circumvent these issues, we developed a photoradiochemical approach that uses fast, chemoselective, light‐induced protein modification under mild conditions with novel metal‐ion‐binding chelates derivatised with aryl azide (ArN₃) groups. Experiments show that one‐pot photochemical conjugation and radiolabelling of formulated mAbs can be achieved in <20 min.     

Enzyme mediated incorporation of zirconium-89 or copper-64 into a fragment antibody for same day imaging of epidermal growth factor receptor

Identification of tumors which over-express Epidermal Growth Factor Receptor (EGFR) is important in selecting patients for anti-EGFR therapies. Enzymatic bioconjugation was used to introduce positron-emitting radionuclides (⁸⁹Zr, ⁶⁴Cu) into an anti-EGFR antibody fragment for Positron Emission Tomography (PET) imaging the same day as injection. A monovalent antibody fragment with high affinity for EGFR was engineered to include a sequence that is recognized by the transpeptidase sortase A. Two different metal chelators, one for ⁸⁹Zr^IV and one for ⁶⁴Cu^II, were modified with a N-terminal glycine to enable them to act as substrates in sortase A mediated bioconjugation to the antibody fragment. Both fragments provided high-quality PET images of EGFR positive tumors in a mouse model at 3 hours post-injection, a significant advantage when compared to radiolabeled full antibodies that require several days between injection of the tracer and imaging. The use of enzymatic bioconjugation gives reproducible homogeneous products with the metal complexes selectively installed on the C-terminus of the antibody potentially simplifying regulatory approval.

Enzymatic bioconjugation to introduce positron-emitting radionuclides (⁸⁹Zr, ⁶⁴Cu) into an anti-EGFR antibody fragment allows same day imaging with positron emission tomography.     

Separation of Y from Sr by zirconium vanadate gel ion-exchanger sorbent: kinetics and thermodynamic study

A new zirconium vanadate (Zr–V) ion-exchanger was synthesized and characterized for fast and selective separation procedure of ⁹⁰Y from ⁸⁹Sr. The method was based on ⁹⁰Y(III) sorption from aqueous HCl solution containing ⁸⁹Sr(II) onto Zr–V gel exchanger. The kinetics of Y(III) sorption from HCl solution by Zr–V exchanger was subjected to Weber–Morris, Lagergren, Bhattacharya and Venkobachar, and Bt models. Initially, the uptake of Y(III) onto the exchanger was fast followed by kinetically first-order sorption with an overall rate constant, K _Lager = (3.55 ± 0.03) × 10^?4 min^?1. Film and intraparticle transport are the two steps that might influence Y(III) sorption. The negative values of ΔG of ⁹⁰Y retention dictate that, the process is a spontaneous. The negative values of ΔH and ΔS reflect the exothermic nature of ⁹⁰Y(IIsorption and the random uptake of ⁹⁰Y(III) onto Zr–V sorbent. Zr–V exchanger offers unique advantages of ⁹⁰Y(III) retention over conventional solid sorbents in rapid and effective separation of traces of ⁹⁰Y(III) from Sr. The exchanger was successfully packed in column for an effective separation of ⁹⁰Y.     

Biocompatible Triplex Ag@SiO2@mTiO2 Core–Shell Nanoparticles for Simultaneous Fluorescence‐SERS Bimodal Imaging and Drug Delivery

Herein, we report the synthesis of biocompatible triplex Ag@SiO₂@mTiO₂ core–shell nanoparticles (NPs) for simultaneous fluorescence‐surface‐enhanced Raman scattering (F‐SERS) bimodal imaging and drug delivery. Stable Raman signals were created by typical SERS tags that were composed of Ag NPs for optical enhancement, a reporter molecule of 4‐mercaptopyridine (4‐Mpy) for a spectroscopic signature, and a silica shell for protection. A further coating of mesoporous titania (mTiO₂) on the SERS tags offered high loading capacity for a fluorescence dye (flavin mononucleotide) and an anti‐cancer drug (doxorubicin (DOX)), thereby endowing the material with fluorescence‐imaging and therapeutic functions. The as‐prepared F‐SERS dots exhibited strong fluorescence when excited by light at 460 nm whilst a stable, characteristic 4‐Mpy SERS signal was detected when the excitation wavelength was changed to longer wavelength (632.8 nm), both in solution and after incorporation inside living cells. Their excellent biocompatibility was demonstrated by low cytotoxicity against MCF‐7 cells, even at a high concentration of 100 μg mL^?1. In vitro cell cytotoxicity confirmed that DOX‐loaded F‐SERS dots had a comparable or even greater therapeutic effect compared with the free drug, owing to the increased cell‐uptake, which was attributed to the possible endocytosis mechanism of the NPs. To the best of our knowledge, this is the first proof‐of‐concept investigation on a multifunctional nanomedicine that possessed a combined capacity for fast and multiplexed F‐SERS labeling as well as drug‐loading for cancer therapy.     

89Zr Labeled Fe3O4@TiO2 Nanoparticles: In Vitro Afffinities with Breast and Prostate Cancer Cells

In this study, Fe₃O₄@TiO₂ nanoparticles were synthesized as a new Positron Emission Tomography/Magnetic Resonance Imaging (PET/MRI) hybrid imaging agent and radiolabeled with ⁸⁹Zr. In addition, Fe₃O₄ nanoparticles were synthesized and radiolabeled with ⁸⁹Zr. Df-Bz-NCS was used as bifunctional ligand. The nanoconjugates were characterized with transmission electron microscopy, scanning electron microscopy, and dynamic light scattering. Radiolabeling yields were 100%. Breast and prostate cancer cell affinities and cytotoxicity were determined using in vitro cell culture assays. The results demonstrated that Fe₃O₄@TiO₂ nanoparticles are promising for PET/MR imaging. Finally, unlike Fe₃O₄ nanoparticles, Fe₃O₄@TiO₂ nanoparticles showed a fluorescence spectrum at an excitation wavelength of 250 nm and an emission wavelength of 314 nm. Therefore, in addition to bearing the magnetic properties of Fe₃O₄ nanoparticles, Fe₃O₄@TiO₂ nanoparticles display fluorescence emission. This provides them with photodynamic therapy potential. Therefore multimodal treatment was performed with the combination of PDT and RT by using human prostate cancer cell line (PC3). The development of ⁸⁹Zr-Df-Bz-NCS-Fe₃O₄@TiO₂ nanoparticles as a new multifunctional PET/MRI agent with photodynamic therapy and hyperthermia therapeutic ability would be very useful.     

Near‐Infrared Light and pH‐Responsive Polypyrrole@Polyacrylic acid/Fluorescent Mesoporous Silica Nanoparticles for Imaging and Chemo‐Photothermal Cancer Therapy

We have rationally designed a new theranostic agent by coating near‐infrared (NIR) light‐absorbing polypyrrole (PPY) with poly(acrylic acid) (PAA), in which PAA acts as a nanoreactor and template, followed by growing small fluorescent silica nanoparticles (fSiO₂ NPs) inside the PAA networks, resulting in the formation of polypyrrole@polyacrylic acid/fluorescent mesoporous silica (PPY@PAA/fmSiO₂) core–shell NPs. Meanwhile, DOX‐loaded PPY@PAA/fmSiO₂ NPs as pH and NIR dual‐sensitive drug delivery vehicles were employed for fluorescence imaging and chemo‐photothermal synergetic therapy in vitro and in vivo. The results demonstrate that the PPY@PAA/fmSiO₂ NPs show high in vivo tumor uptake by the enhanced permeability and retention (EPR) effect after intravenous injection as revealed by in vivo fluorescence imaging, which is very helpful for visualizing the location of the tumor. Moreover, the obtained NPs inhibit tumor growth (95.6 % of tumors were eliminated) because of the combination of chemo‐photothermal therapy, which offers a synergistically improved therapeutic outcome compared with the use of either therapy alone. Therefore, the present study provides new insights into developing NIR and pH‐stimuli responsive PPY‐based multifunctional platform for cancer theranostics.     

A Robust Infinite Zirconium Phenolate Building Unit to Enhance the Chemical Stability of Zr MOFs

A novel Zr‐chain based MOF, namely MIL‐163, was designed and successfully synthesized using a bis‐1,2,3‐trioxobenzene ligand. Endowed with large square‐shaped channels of 12 Å width, it shows remarkable water uptake (ca. 0.6 cm³ g^?1 at saturating vapor pressure) and a remarkable stability in simulated physiological media, where archetypical Zr carboxylate MOFs readily degrade.     

Optimization of 89Zr production using Monte Carlo simulations

Zirconium-89 (⁸⁹Zr) can be produced in a cyclotron by focusing the proton beam on an yttrium-89 (⁸⁹Y) foil target. Optimal combination of beam energy and target assembly configuration enables maximum production of ⁸⁹Zr while minimizing the formation of contaminant nuclides such as ⁸⁸Zr and ⁸⁸Y to allow efficient and effective radiopharmaceutical labeling. Accurate modeling of the proton beam and the target is therefore an essential step to assure the best beam and target specification. We used the radiation transport code MCNPX to simulate the transport of protons through the irradiation assembly and the nuclear reaction code TALYS to obtain the production cross sections of various nuclides from proton-⁸⁹Y reactions. Results from simulating the irradiation of 14 mm diameter targets with aluminum (Al) degrader at 19.8 mA for 1 h suggested that the 0.15 mm thick one would produce 227 MBq while the 0.3 mm thick one would produce 413 MBq of ⁸⁹Zr with less than 1 % uncertainty. These results show excellent agreement with experimental work in literature. This work provides the basis for further experimental and theoretical assessments of the use of ⁸⁹Zr as radiopharmaceutical labels.     

Silica‐Encapsulated Gd3+‐Aggregated Gold Nanoclusters for In Vitro and In Vivo Multimodal Cancer Imaging

Gd³⁺‐aggregated gold nanoclusters (AuNCs) encapsulated by silica shell (Gd³⁺‐A‐AuNCs@SiO₂ NPs) were strategically designed and prepared. The as‐prepared nanoparticles exhibit aggregation‐enhanced fluorescence (AEF), with an intensity that is up to 3.8 times that of discrete AuNCs. The clusters served as novel nanoprobes for in vitro and in vivo multimodal (fluorescence, magnetic resonance, and computed X‐ray tomography) cancer imaging     

Mussel‐Inspired Protein Nanoparticles Containing Iron(III)–DOPA Complexes for pH‐Responsive Drug Delivery

A novel bioinspired strategy for protein nanoparticle (NP) synthesis to achieve pH‐responsive drug release exploits the pH‐dependent changes in the coordination stoichiometry of iron(III)–3,4‐dihydroxyphenylalanine (DOPA) complexes, which play a major cross‐linking role in mussel byssal threads. Doxorubicin‐loaded polymeric NPs that are based on Fe^III–DOPA complexation were thus synthesized with a DOPA‐modified recombinant mussel adhesive protein through a co‐electrospraying process. The release of doxorubicin was found to be predominantly governed by a change in the structure of the Fe^III–DOPA complexes induced by an acidic pH value. It was also demonstrated that the fabricated NPs exhibited effective cytotoxicity towards cancer cells through efficient cellular uptake and cytosolic release. Therefore, it is anticipated that Fe^III–DOPA complexation can be successfully utilized as a new design principle for pH‐responsive NPs for diverse controlled drug‐delivery applications.     

Rare‐Earth‐Based Nanoparticles with Simultaneously Enhanced Near‐Infrared (NIR)‐Visible (Vis) and NIR‐NIR Dual‐Conversion Luminescence for Multimodal Imaging

Multifunctional NaGdF₄:Yb³⁺,Er³⁺,Nd³⁺@NaGdF₄:Nd³⁺ core–shell nanoparticles (called Gd:Yb³⁺,Er³⁺,Nd³⁺@Gd:Nd³⁺ NPs) with simultaneously enhanced near‐infrared (NIR)‐visible (Vis) and NIR‐NIR dual‐conversion (up and down) luminescence (UCL/DCL) properties were successfully synthesized. The resulting core–shell NPs simultaneously emitted enhanced UCL at 522, 540, and 660 nm and DCL at 980 and 1060 nm under the excitation of a 793 nm laser. The enhanced UCL and DCL can be explained by complex energy‐transfer processes, Nd³⁺→Yb³⁺→Er³⁺ and Nd³⁺→Yb³⁺, respectively. The effects of Nd³⁺ concentration and shell thickness on the UCL/DCL properties were systematically investigated. The UCL and DCL properties of NPs were observed under the optimal conditions: a shell Nd³⁺ content of 20 % and a shell thickness of approximately 5 nm. Moreover, the Gd:Yb³⁺,Er³⁺,Nd³⁺@Gd:20 % Nd³⁺ NPs exhibited remarkable magnetic resonance imaging (MRI) properties similar to that of a clinical agent, Omniscan. Thus, the core–shell NPs with excellent UCL/DCL/magnetic resonance imaging (MRI) properties have great potential for both in vitro and in vivo multimodal bioimaging.     

One‐Step Synthesis of Small‐Sized and Water‐Soluble NaREF4 Upconversion Nanoparticles for In Vitro Cell Imaging and Drug Delivery

Small (2–28 nm) NaREF₄ (rare earth (RE)=Nd–Lu, Y) nanoparticles (NPs) were prepared by an oil/water two‐phase approach. Meanwhile, hydrophilic NPs can be obtained through a successful phase‐transition process by introducing the amphiphilic surfactant sodium dodecylsulfate (SDS) into the same reaction system. Hollow‐structured NaREF₄ (RE=Y, Yb, Lu) NPs can be fabricated in situ by electron‐beam lithography on solid NPs. The MTT assay indicates that these hydrophilic NPs with hollow structures exhibit good biocompatibility. The as‐prepared hollow‐structured NPs can be used as anti‐cancer drug carriers for drug storage/release investigations. Doxorubicin hydrochloride (DOX) was taken as model drug. The release of DOX from hollow α‐NaLuF₄:20 % Yb³⁺, 2 % Er³⁺ exhibits a pH‐sensitive release patterns. Confocal microscopy observations indicate that the NPs can be taken up by HeLa cells and show obvious anti‐cancer efficacy. Furthermore, α‐NaLuF₄:20 % Yb³⁺, 2 % Er³⁺ NPs show bright‐red emission under IR excitation, making both the excitation and emission light fall within the “optical window” of biological tissues. The application of α‐NaLuF₄:20 % Yb³⁺, 2 % Er³⁺ in the luminescence imaging of cells was also investigated, which shows a bright‐red emission without background noise.