Advances in functional X-ray imaging techniques and contrast agents

X-ray fluorescence imaging of metals and metalloids in biological systems

Metals and metalloids play fundamental roles in many physiological processes in biological systems, but imbalance of these elements in the body may cause many diseases, such as Parkinson's disease, Alzheimer's disease, and even cancers. Thus, to better understand the metallome in health and disease, quantitative determination of their localization, concentration, speciation, and related metabolism at cellular or subcellular levels is of great importance. X-ray fluorescence (XRF) imaging, as a new generation of analytical technique, has been reported as an ideal tool to quantitatively map multiple metals and metalloids in tissues with reasonable sensitivity, specificity, and resolution. In the current review, we have introduced the general concept of XRF imaging technique, reviewed the recent advances using XRF imaging to investigate toxicology of metals and metalloids in life science, and discussed the roles of metals and metalloids in various diseases, including cancers and neurodegenerative diseases. We believe that future research on revealing the roles of metals and metalloids in biological systems will directly benefit from the important breakthroughs in developing XRF imaging techniques.

Microbubbles containing gadolinium as contrast agents for both phase contrast and magnetic resonance imaging

Portal vein imaging is an important method for investigating portal venous disorders. However, the diagnostic requirements are not usually satisfied when using single imaging techniques. Diagnostic accuracy can be improved by combining different imaging techniques. Contrast agents that can be used for combined imaging modalities are needed. In this study, the feasibility of using microbubbles containing gadolinium (MCG) as contrast agents for both phase contrast imaging (PCI) and magnetic resonance imaging (MRI) are investigated. MCG were made by encapsulating sulfur hexafluoride (SF₆) gas with gadolinium and lyophilized powder. Absorption contrast imaging (ACI) and PCI of MCG were performed and compared in vitro. MCG were injected into the main portal trunk of living rats. PCI and MRI were performed at 2 min and 10 min after MCG injection, respectively. PCI exploited the differences in the refractive index and visibly showed the MCG, which were not detectable by ACI. PCI could facilitate clear revelation of the MCG-infused portal veins. The diameter of the portal veins could be determined by the largest MCG in the same portal vein. The minimum diameter of clearly detected portal veins was about 300 µm by MRI. These results indicate that MCG could enhance both PCI and MRI for imaging portal veins. The detection sensitivity of PCI and MRI could compensate for each other when using MCG contrast agents for animals.

Cone-beam x-ray luminescence computed tomography based on x-ray absorption dosage

With the advances of x-ray excitable nanophosphors, x-ray luminescence computed tomography (XLCT) has become a promising hybrid imaging modality. In particular, a cone-beam XLCT (CB-XLCT) system has demonstrated its potential in in vivo imaging with the advantage of fast imaging speed over other XLCT systems. Currently, the imaging models of most XLCT systems assume that nanophosphors emit light based on the intensity distribution of x-ray within the object, not completely reflecting the nature of the x-ray excitation process. To improve the imaging quality of CB-XLCT, an imaging model that adopts an excitation model of nanophosphors based on x-ray absorption dosage is proposed in this study. To solve the ill-posed inverse problem, a reconstruction algorithm that combines the adaptive Tikhonov regularization method with the imaging model is implemented for CB-XLCT reconstruction. Numerical simulations and phantom experiments indicate that compared with the traditional forward model based on x-ray intensity, the proposed dose-based model could improve the image quality of CB-XLCT significantly in terms of target shape, localization accuracy, and image contrast. In addition, the proposed model behaves better in distinguishing closer targets, demonstrating its advantage in improving spatial resolution.

Thermometry properties of Er, Yb-Gd2O2S microparticles: dependence on the excitation mode (cw versus pulsed excitation) and excitation wavelength (980 nm versus 1500 nm)

Herein, we present a first report on the luminescence thermometry properties of Er, Yb doped Gd₂O₂S microparticles under near infrared up-conversion excitation at 980 and 1500 nm measured in the 280-800 K interval. The thermometry properties are assessed using both cw and ns pulsed excitation as well as tuning the excitation wavelength across Yb and Er absorption profiles. For low cw (300 mW cm^-1) and pulsed ns (400 ÷ 550 mW cm^-1) excitation modes, no thermal load is observed. At room-temperature (280 K), the maximum relative sensitivity values are comparable under pulsed excitation at 980 and 1500 nm, around ∼0.01 and ∼0.008% K^-1, respectively. In addition, a relative intense up-conversion emission at 980 nm under excitation at 1500 nm is measured. Our findings evidence attractive up-conversion and thermometry properties Er, Yb doped Gd₂O₂S under near-infrared excitation and highlight the need to explore further these properties in the nanoparticulate regime.

Sub-10 nm Water-Dispersible β-NaGdF4:X% Eu3+ Nanoparticles with Enhanced Biocompatibility for in Vivo X-ray Luminescence Computed Tomography

As a novel molecular and functional imaging modality, X-ray luminescence computed tomography (XLCT) has shown its potentials in biomedical and preclinic applications. However, there are still some limitations of X-ray-excited luminescent materials, such as low luminescence efficiency, poor biocompatibility, and cytotoxicity, making in vivo XLCT imaging quite challenging. In this study, for the very first time, we present on using sub-10 nm β-NaGdF₄:X% Eu³⁺ nanoparticles with poly(acrylic acid) (PAA) surface modification, which demonstrate outstanding luminescence efficiency, uniform size distribution, water dispersity, and biosafety, as the luminescent probes for in vivo XLCT application. The pure hexagonal phase (β-) NaGdF₄ has been successfully synthesized and characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM), and then the results of X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectrometry  (EDX), and elemental mapping further confirm Eu³⁺ ions doped into NaGdF₄ host. Under X-ray excitation, the β-NaGdF₄ nanoparticles with a doping level of 15% Eu³⁺ exhibited the most efficient luminescence intensity. Notably, the doping level of Eu³⁺ has no effect on the crystal phase and morphology of the NaGdF₄-based host. Afterward, β-NaGdF₄:15% Eu³⁺ nanoparticles were modified with PAA to enhance the water dispersity and biocompatibility. The compatibility of in vivo XLCT imaging using such nanoparticles was systematically studied via in vitro cytotoxicity, physical phantom, and in vivo imaging experiments. The ultralow cytotoxicity of PAA-modified nanoparticles, which is confirmed by over 80% cell viability of SH-SY5Y cells when treated by high nanoparticle concentration of 200 μg/mL, overcome the major obstacle for in vivo application. In addition, the high luminescence intensity of PAA-modified nanoparticles enables the location error of in vivo XLCT imaging less than 2 mm, which is comparable to that using commercially available bulk material Y₂O₃:15% Eu³⁺. The proposed nanoparticles promote XLCT research into an in vivo stage. Further modification of these nanoparticles with biofunctional molecules could enable the potential of targeting XLCT imaging.

LiGa5O8:Cr-based theranostic nanoparticles for imaging-guided X-ray induced photodynamic therapy of deep-seated tumors

Using X-ray as the irradiation source, a photodynamic therapy process can be initiated from under deep tissues. This technology, referred to as X-ray induced PDT, or X-PDT, holds great potential to treat tumors at internal organs. To this end, one question is how to navigate the treatment to tumors with accuracy with external irradiation. Herein we address the issue with a novel, LiGa₅O₈: Cr (LGO:Cr)-based nanoscintillator, which emits persistent, near-infrared X-ray luminescence. This permits deep-tissue optical imaging that can be employed to guide irradiation. Specifically, we encapsulated LGO:Cr nanoparticles and a photosensitizer, 2,3-naphthalocyanine, into mesoporous silica nanoparticles. The nanoparticles were conjugated with cetuximab and systemically injected into H1299 orthotopic non-small cell lung cancer tumor models. The nanoconjugates can efficiently home to tumors in the lung, confirmed by monitoring X-ray luminescence from LGO:Cr. Guided by the imaging, external irradiation was applied, leading to efficient tumor suppression while minimally affecting normal tissues. To the best of our knowledge, the present study is the first to demonstrate, with systematically injected nanoparticles, that X-PDT can suppress growth of deep-seated tumors. The imaging guidance is also new to X-PDT, and is significant to the further transformation of the technology.

Organic-Inorganic Composite Films Based on Gd3Ga3Al2O12:Ce Scintillator Nanoparticles for X-ray Imaging Applications

Organic-inorganic nanocomposite self-standing films of Gd₃Ga₃Al₂O₁₂ (GGAG) uniformly dispersed in poly(methyl methacrylate) (PMMA) and polystyrene polymer are prepared for radiography application. GGAG:Ce nanoscintillator has been chosen because of its high light output and fast decay time. The nanopowder of GGAG is synthesized by coprecipitation method and dispersed in the polymer matrix by a simple blending technique. The nanocomposite films of thickness in the range of 150-450 μm with a very high inorganic content is achieved by this technique. These films are characterized by their uniformity, optical absorption, photoluminescence, and radioluminescence. These films are further tested for their application in radiography by recording X-ray images using a commercially available charge-coupled device camera. A resolution of 10 lp/mm is obtained using GGAG:PMMA composite film with 50% loading, confirming their application in imaging devices.

Bright X-ray and up-conversion nanophosphors annealed using encapsulated sintering agents for bioimaging applications

Nanophosphors are promising contrast agents for deep tissue optical imaging applications because they can be excited by X-ray and near infrared light that penetrates deeply through tissue and generates almost no autofluorescence background in the tissue. For these bioimaging applications, the nanophosophors should ideally be small, monodispersed and brightly luminescent. However, most methods used to improve luminescence yield by annealing the particles to reduce crystal and surface defects (e.g. using flux or sintering agents) also cause particle fusion or require multiple component core-shell structures. Here, we report a novel method to prepare bright, uniformly sized X-ray nanophosphors (Gd2O2S:Eu or Tb) and upconversion nanophosphors (Y2O2S: Yb/Er, or Yb/Tm) with large crystal domain size without causing aggregation. A core-shell nanoparticle is formed, with NaF only in the core. We observe that increasing the NaF sintering agent concentration up to 7.6 mol% increases both crystal domain size and luminescence intensity (up to 40% of commercial microphosphors) without affecting the physical particticle diameter. Above 7.6 mol%, particle fusion is observed. The annealing is insensitive to the cation (Na+ or K+) but varies strongly with anion, with F->Cl->CO32->Br->I-. The luminescence depends strongly on crystal domain size. The data agree reasonably well with a simple domain surface quenching model, although the size-dependence suggests additional quenching mechanisms within small domains. The prepared bright nanophosphors were subsequently functionalized with PEG-folic acid to target MCF-7 breast cancer cells which overexpress folic acid receptors. Both X-ray and upconversion nanophosphors provided low background and bright luminescence which was imaged through 1 cm chicken breast tissue at a low dose of nanophosphors 200 µL (0.1 mg/mL). We anticipate these highly monodispersed and bright X-ray and upconversion nanophosphors will have significant potential for tumor targeted imaging.

A comparative study of optical properties and X-ray induced luminescence of octahedral molybdenum and tungsten cluster complexes

Octahedral W cluster complexes have more intensive X-ray excited optical luminescence than Mo ones.

X-ray excited luminescent chemical imaging (XELCI) for non-invasive imaging of implant infections

X-ray excited luminescent chemical imaging (XELCI) uses a combination of X-ray excitation to provide high resolution and optical detection to provide chemical sensing. A key application is to detect and study implant-associated infection. The implant is coated with a layer of X-ray scintillators which generate visible near infrared light when irradiated with an X-ray beam. This light first passes through a pH indicator dye-loaded film placed over the scintillator film in order to modulate the luminescence spectrum according to pH. The light then passes through tissue is collected and the spectral ratio measured to determine pH. A focused X-ray beam irradiates a point in the scintillator film, and a pH image is formed point-by-point by scanning the beam across the sample. The sensor and scanning system are described along with preliminary results showing images in rabbit models.

Reactive oxygen species generating systems meeting challenges of photodynamic cancer therapy

The reactive oxygen species (ROS)-mediated mechanism is the major cause underlying the efficacy of photodynamic therapy (PDT). The PDT procedure is based on the cascade of synergistic effects between light, a photosensitizer (PS) and oxygen, which greatly favors the spatiotemporal control of the treatment. This procedure has also evoked several unresolved challenges at different levels including (i) the limited penetration depth of light, which restricts traditional PDT to superficial tumours; (ii) oxygen reliance does not allow PDT treatment of hypoxic tumours; (iii) light can complicate the phototherapeutic outcomes because of the concurrent heat generation; (iv) specific delivery of PSs to sub-cellular organelles for exerting effective toxicity remains an issue; and (v) side effects from undesirable white-light activation and self-catalysation of traditional PSs. Recent advances in nanotechnology and nanomedicine have provided new opportunities to develop ROS-generating systems through photodynamic or non-photodynamic procedures while tackling the challenges of the current PDT approaches. In this review, we summarize the current status and discuss the possible opportunities for ROS generation for cancer therapy. We hope this review will spur pre-clinical research and clinical practice for ROS-mediated tumour treatments.

Multimodal Imaging of Nanocomposite Microspheres for Transcatheter Intra-Arterial Drug Delivery to Liver Tumors

A modern multi-functional drug carrier is critically needed to improve the efficacy of image-guided catheter-directed approaches for the treatment of hepatic malignancies. For this purpose, a nanocomposite microsphere platform was developed for selective intra-arterial transcatheter drug delivery to liver tumors. In our study, continuous microfluidic methods were used to fabricate drug-loaded multimodal MRI/CT visible microspheres that included both gold nanorods and magnetic clusters. The resulting hydrophilic, deformable, and non-aggregated microspheres were mono-disperse and roughly 25 um in size. Sustained drug release and strong MRI T₂ and CT contrast effects were achieved with the embedded magnetic nano-clusters and radiopaque gold nanorods. The microspheres were successfully infused through catheters selectively placed within the hepatic artery in rodent models and subsequent distribution in the targeted liver tissues and hepatic tumors confirmed with MRI and CT imaging. These multimodal nanocomposite drug carriers should be ideal for selective intra-arterial catheter-directed administration to liver tumors while permitting MRI/CT visualization for patient-specific confirmation of tumor-targeted delivery.

Scintillating Nanoparticles as Energy Mediators for Enhanced Photodynamic Therapy

Achieving effective treatment of deep-seated tumors is a major challenge for traditional photodynamic therapy (PDT) due to difficulties in delivering light into the subsurface. Thanks to their great tissue penetration, X-rays hold the potential to become an ideal excitation source for activating photosensitizers (PS) that accumulate in deep tumor tissue. Recently, a wide variety of nanoparticles have been developed for this purpose. The nanoparticles are designed as carriers for loading various kinds of PSs and can facilitate the activation process by transferring energy harvested from X-ray irradiation to the loaded PS. In this review, we focus on recent developments of nanoscintillators with high energy transfer efficiency, their rational designs, as well as potential applications in next-generation PDT. Treatment of deep-seated tumors by using radioisotopes as an internal light source will also be discussed.

[The Development of Luminescent Nano-probes on Hard X-ray Irradiation]

  X-rays are widely used in imaging applications such as diffraction imaging of crystals and medical imaging. In particular, X-ray computed tomography (CT) is a critical tool for clinical and disease diagnostics. The principle of conventional CT is based on X-ray attenuation caused by photoelectric absorption and scattering. In addition to conventional CT, a number of novel methodologies are presently under development, including state-of-the-art instrument technologies and chemical probes to fulfill diagnosis criteria. Among these novel methodologies, we have utilized hard X-ray-excited optical luminescence (hXEOL) as a new methodology to enhance the contrast of the image. Herein, we explored the possibility of hXEOL via iridium-doped polymer nanoparticles and biomolecule-directed metal clusters and propose it as a potential platform for new X-ray imaging.

Nanoscintillator-mediated X-ray inducible photodynamic therapy for in vivo cancer treatment

Photodynamic therapy is a promising treatment method, but its applications are limited by the shallow penetration of visible light. Here, we report a novel X-ray inducible photodynamic therapy (X-PDT) approach that allows PDT to be regulated by X-rays. Upon X-ray irradiation, the integrated nanosystem, comprised of a core of a nanoscintillator and a mesoporous silica coating loaded with photosensitizers, converts X-ray photons to visible photons to activate the photosensitizers and cause efficient tumor shrinkage.

Detecting De-gelation through Tissue Using Magnetically Modulated Optical Nanoprobes (MagMOONs)

Alginate gels are widely used for drug delivery and implanted devices. The rate at which these gels break down is important for controlling drug release. Since the de-gelation may be different in vivo, monitoring this process in situ is essential. However, it is challenging to monitor the gel through tissue due to optical scattering and tissue autofluorescence. Herein we describe a method to detect through tissue the chemically-induced changes in viscosity and de-gelation process of alginate gels using magnetically modulated optical nanoprobes (MagMOONs). The MagMOONs are fluorescent magnetic microspheres coated with a thin layer of opaque metal on one hemisphere. The metal layer prevents excitation and emission light from passing through one side of the MagMOONs, which creates orientation-dependent fluorescence intensity. The magnetic particles also align in an external magnetic field and give blinking signals when they rotate to follow an external modulated magnetic field. The blinking signals from these MagMOONs are distinguished from background autofluorescence and can be tracked on a single particle level in the absence of tissue, or for an ensemble average of particles blinking through tissue. When these MagMOONs are dispersed in calcium alginate gel, they become sensors for detecting gel degradation upon addition of either ammonium ion or alginate lyase. Our results show MagMOONs start blinking approximately 10 minutes after 2 mg/mL alginate lyase addition and this blinking is clearly detected even through up to 4 mm chicken breast. This approach can potentially be employed to detect bacterial biofilm formation on medical implants by sensing specific proteases that either activate a related function or regulate biofilm formation. It can also be applied to other biosensors and drug delivery systems based on enzyme-catalyzed breakdown of gel components.

Synchrotron-based X-ray-sensitive nanoprobes for cellular imaging

It is one of the ultimate goals in cell biology to understand the complex spatio-temporal interplay of biomolecules in the cellular context. To this end, there have been great efforts on the development of various probes to detect and localize specific biomolecules in cells with a variety of microscopic imaging techniques. In this Research News, we first summarize several types of microscopy for visualizing specific biomolecular targets. Then we focus on recent advances in the design of X-ray sensitive nanoprobes for applications in synchrotron-based cellular imaging. With the availability of advanced synchrotron techniques, there has been rapid progress toward high-resolution and multi-color X-ray imaging in cells with various types of functional nanoprobes.

Hard X-ray-induced optical luminescence via biomolecule-directed metal clusters

Here, we demonstrate that biomolecule-directed metal clusters are applicable in the study of hard X-ray excited optical luminescence, promising a new direction in the development of novel X-ray-activated imaging probes.

Chitosan derivatives cross-linked with iodinated 2,5-dimethoxy-2,5-dihydrofuran for non-invasive imaging

Radiopaque polymer derivatives were successfully prepared through surface diffusion mediated cross-linking of chitosan with iodinated 2,5-dimethoxy-2,5-dihydrofuran. The incorporation of iodine in 2,5-dimethoxy-2,5-dihydrofuran was validated by (1)H NMR and mass spectroscopy. The cross-linking of the glucosamine moieties of chitosan with the iodinated product was confirmed by (13)C NMR and energy-dispersive X-ray spectroscopy. Radiography analysis proved inherent opacity of the iodinated fibrous sheets and microspheres that were comparable to the X-ray visibility of aluminum hollow rings of equivalent thickness and commercially available radiopaque tape, respectively. Microscopic studies evidenced retention of the fiber/microsphere morphology after the iodination/cross-linking reactions. The effects of iodination/cross-linking on the mechanical and biodegradation properties of fibers were studied by nanoindentation and enzymatic assay, respectively. In vitro and in vivo studies established the nontoxic, biodegradable nature of radiopaque derivatives. Iodinated fiber mesh implanted in a rabbit model was significantly X-ray opaque compared to the uncross-linked fiber mesh and medical grade surgical swabs. Further, opacity of the iodinated mesh was evident even after 60 days, though the intensity was reduced, which indicates the biodegradable nature of the iodinated polymer. The opacity of the iodinated sutures was also established in the computed tomography images. Finally, the sufficient in vivo contrast property of the radiopaque microspheres in the gastrointestinal tract indicates its possible role in clinical diagnostics.

Multifunctional yolk-in-shell nanoparticles for pH-triggered drug release and imaging

Multifunctional nanoparticles are synthesized for both pH-triggered drug release and imaging with radioluminescence, upconversion luminescent, and magnetic resonance imaging (MRI). The particles have a yolk-in-shell morphology, with a radioluminescent core, an upconverting shell, and a hollow region between the core and shell for loading drugs. They are synthesized by controlled encapsulation of a radioluminescent nanophosphor yolk in a silica shell, partial etching of the yolk in acid, and encapsulation of the silica with an upconverting luminescent shell. Metroxantrone, a chemotherapy drug, was loaded into the hollow space between X-ray phosphor yolk and up-conversion phosphor shell through pores in the shell. To encapsulate the drug and control the release rate, the nanoparticles are coated with pH-responsive biocompatible polyelectrolyte layers of charged hyaluronic acid sodium salt and chitosan. The nanophosphors display bright luminescence under X-ray, blue light (480 nm), and near infrared light (980 nm). They also served as T1 and T2 MRI contrast agents with relaxivities of 3.5 mM(-1) s(-1) (r1 ) and 64 mM(-1) s(-1) (r2 ). These multifunctional nanocapsules have applications in controlled drug delivery and multimodal imaging.

NaGdF4:Eu3+ Nanoparticles for Enhanced X-ray Excited Optical Imaging

X-ray luminescent nanoparticles (NPs), including lanthanide fluorides, have been evaluated for application to deep tissue in vivo molecular imaging using optical tomography. A combination of high material density, higher atomic number and efficient NIR luminescence from compatible lanthanide dopant ions indicates that particles that consist of ALnF₄ (A = alkaline, Ln = lanthanide element) may offer a very attractive class of materials for high resolution, deep tissue imaging with X-ray excitation. NaGdF₄:Eu³⁺ NPs produced an X-ray excited luminescence that was among the most efficient of nanomaterials that have been studied thus far. We have systematically studied factors such as (a) the crystal structure that changes the lattice environment of the doped Eu³⁺ ions within the unit cell; and extrinsic factors such as (b) a gold coating (with attendant biocompatibility) that couples to a plasmonic excitation, and (c) changes in the NPs surface properties via changes in the pH of the suspending medium-all with a significant impact on the X-ray excited luminescence of NaGdF₄:Eu³⁺NPs. The luminescence from an optimally doped hexagonal phase NaGdF₄:Eu³⁺ nanoparticle was 25% more intense compared to that of a cubic structure. We observed evidence of plasmonic reabsorption of midwavelength emission by a gold coating on hexagonal NaGdF₄:Eu³⁺ NPs; fortunately, the NaGdF₄:Eu³⁺ @Au core-shell NPs retained the efficient ⁵D₀→⁷F₄ NIR (692 nm) luminescence. The NaGdF₄:Eu³⁺ NPs exhibited sensitivity to the ambient pH when excited by X-rays, an effect not seen with UV excitation. The sensitivity to the local environment can be understood in terms of the sensitivity of the excitons that are generated by the high energy X-rays (and not by UV photons) to crystal structure and to the surface state of the particles.

Estimating structure quality trends in the Protein Data Bank by equivalent resolution

The quality of protein structures obtained by different experimental and ab-initio calculation methods varies considerably. The methods have been evolving over time by improving both experimental designs and computational techniques, and since the primary aim of these developments is the procurement of reliable and high-quality data, better techniques resulted on average in an evolution toward higher quality structures in the Protein Data Bank (PDB). Each method leaves a specific quantitative and qualitative "trace" in the PDB entry. Certain information relevant to one method (e.g. dynamics for NMR) may be lacking for another method. Furthermore, some standard measures of quality for one method cannot be calculated for other experimental methods, e.g. crystal resolution or NMR bundle RMSD. Consequently, structures are classified in the PDB by the method used. Here we introduce a method to estimate a measure of equivalent X-ray resolution (e-resolution), expressed in units of Å, to assess the quality of any type of monomeric, single-chain protein structure, irrespective of the experimental structure determination method. We showed and compared the trends in the quality of structures in the Protein Data Bank over the last two decades for five different experimental techniques, excluding theoretical structure predictions. We observed that as new methods are introduced, they undergo a rapid method development evolution: within several years the e-resolution score becomes similar for structures obtained from the five methods and they improve from initially poor performance to acceptable quality, comparable with previously established methods, the performance of which is essentially stable.

Monitoring pH-triggered drug release from radioluminescent nanocapsules with X-ray excited optical luminescence

One of the greatest challenges in cancer therapy is to develop methods to deliver chemotherapy agents to tumor cells while reducing systemic toxicity to noncancerous cells. A promising approach to localizing drug release is to employ drug-loaded nanoparticles with coatings that release the drugs only in the presence of specific triggers found in the target cells such as pH, enzymes, or light. However, many parameters affect the nanoparticle distribution and drug release rate, and it is difficult to quantify drug release in situ. In this work, we show proof-of-principle for a "smart" radioluminescent nanocapsule with an X-ray excited optical luminescence (XEOL) spectrum that changes during release of the optically absorbing chemotherapy drug, doxorubicin. XEOL provides an almost background-free luminescent signal for measuring drug release from particles irradiated by a narrow X-ray beam. We study in vitro pH-triggered release rates of doxorubicin from nanocapsules coated with a pH-responsive polyelectrolyte multilayer using HPLC and XEOL spectroscopy. The doxorubicin was loaded to over 5% by weight and released from the capsule with a time constant in vitro of ∼36 days at pH 7.4 and 21 h at pH 5.0, respectively. The Gd₂O₂S:Eu nanocapsules are also paramagnetic at room temperature with similar magnetic susceptibility and similarly good MRI T₂ relaxivities to Gd₂O₃, but the sulfur increases the radioluminescence intensity and shifts the spectrum. Empty nanocapsules did not affect cell viability up to concentrations of at least 250 μg/mL. These empty nanocapsules accumulated in a mouse liver and spleen following tail vein injection and could be observed in vivo using XEOL. The particles are synthesized with a versatile template synthesis technique which allows for control of particle size and shape. The XEOL analysis technique opens the door to noninvasive quantification of drug release as a function of nanoparticle size, shape, surface chemistry, and tissue type.

Dendrimer–folate–copper conjugates as bioprobes for synchrotron X-ray fluorescence imaging

We present a bioprobe for synchrotron X-ray fluorescence imaging based on dendrimer–folate–copper conjugates, which exhibit excellent FR-targeting properties in KB cells.