Optical Bioimaging Research Articles

High-sensitivity imaging of time-domain near-infrared light transducer

The optically transparent biological window in the near-infrared (NIR) spectral range allows deep-tissue excitation and the detection of fluorescence signals1,2. Spectrum-domain discrimination of NIR contrast agents via an upconversion or downshifting scheme requires sufficient (anti-) Stokes shift to separate excitation and fluorescence emission. Here, we report a time-domain (τ) scheme in which about 5,000 ytterbium signal transducers are condensed within an optically inert and biocompatible CaF2 shell (2.3 nm), which forms a 14.5 nm τ-dot. Because of the long-lived and spectrally narrowly defined excited state of pure ytterbium ions, the NIR τ-dot can convert the NIR pulsed excitation into long-decaying luminescence with an efficiency approaching 100%. Within a safe injection dosage of 13 μg g−1, an excitation power density of 1.1 mW cm−2 was sufficient to image organs with a signal-to-noise ratio of >9. The high brightness of τ-dots further allows long-term in vivo passive targeting and dynamic tracking in a tumour-bearing mouse model. By time-shifting short-pulse excitation photon energy into prolonged luminescent emission in the time domain, both the number of light signal transducers in sub-15 nm nanoparticles and the near-infrared-in to near-infrared-out conversion efficiency can be maximized, advancing in vivo optical bioimaging.

In Vivo Assembly and Disassembly of Probes to Improve Near-Infrared Optical Bioimaging.

The near-infrared range (NIR, 700-1700 nm) has been used as a superior optical window for non-invasive bioimaging. Increasing signal-to-noise ratio (SNR) is the most fundamental method to improve NIR bioimaging. However, the low delivery efficiency of fluorescent contrast agents leads to weak signal at lesions. Moreover, non-specific accumulation and "always on" signals will cause "false positive" signals and high background noise, all of which result in low SNR and potential long-term biotoxicity. Thus, to reach precise detection of lesions, strong bioimaging signals and low background interference are the two important pre-requisites. This review provides an overview of in vivo assembly and disassembly strategies to improve tumor-specific accumulation, "turn-on" the silent signals, and reduce the background noise in NIR bioimaging windows. In vivo assembly and disassembly occurring spontaneously, responding to disease micro-environment or external stimuli, including pH, enzymes, reactive oxygen species, redox, light, and specific recognition is summarized, which may provide ideas and approaches to further enhance bioimaging and reduce long-term biotoxicity concerns.

Construction of lanthanide-doped upconversion nanoparticle-Uelx Europaeus Agglutinin-I bioconjugates with brightness red emission for ultrasensitive in vivo imaging of colorectal tumor

Lanthanide-doped upconversion nanoparticles (UCNPs)-based active targeting optical bioimaging has attracted tremendous scientific interest because of its noninvasive real-time signal feedback, superior tissue penetration depth and high spatial resolution in early diagnosis of disease. Herein, we synthesize a novel carboxy-terminated silica coated NaErF4: 10% Yb@NaYF4: 40% Yb@NaNdF4: 10% Yb@NaGdF4: 20% Yb UCNPs (termed as UCNP@SiO2-COOH) with 808 nm near-infrared (NIR) excitation and bright 655 nm upconversion luminescence (UCL) emission for realizing deep tissue imaging. Under 808 nm NIR laser excitation (1.5 W cm−2), the UCL of UCNP@SiO2-COOH with relative low concentration (2 mg mL−1) can be successfully visualized under a chicken breast slice with 10 mm thickness. After conjugated with various molecules including NH2-PEG3400-COOH, peptide D-SP5 and Uelx Europaeus Agglutinin-I (UEA-I), biodistributions, clearance pathways and tumor-targeting capacities of the UCNP@SiO2-COOH and corresponding bioconjugates (termed as UCNP@SiO2-PEG, UCNP@SiO2-D-SP5 and UCNP@SiO2-UEA-I, respectively) were investigated by tracking the UCL intensities of livers, kidneys and tumors. Both of in vitro and in vivo experimental results reveal that there is no significant difference for their in vivo biodistributions and clearance pathways. The UCNP@SiO2-UEA-I exhibits much higher SW480 tumor-targeting capacity than those of other bioconjugates. In particular, the as-prepared UCNP@SiO2-UEA-I even to visualize ultrasmall (c.a. 3 mm3 in volume) subcutaneous SW480 tumor in Balb/c nude mouse through intravenous administration. The study implies that the red UCL emitted UCNPs with a minimized heating effect is suitable for deep tissue biomedical imaging and UCNP@SiO2-UEA-I can serve as an efficient optical probe for early diagnosis of SW480 tumor.

Bi‐Modal Nonlinear Optical Contrast from Si Nanoparticles for Cancer Theranostics

AbstractPresenting a safe alternative to conventional compound quantum dots and other functional nanostructures, nanosilicon can offer a series of breakthrough hyperthermia‐based therapies under near‐infrared, radiofrequency, ultrasound, etc., excitation, but the size range to sensitize these therapies is typically too large (>10 nm) to enable efficient imaging functionality based on photoluminescence properties of quantum‐confined excitonic states. Here, it is shown that large Si nanoparticles (NPs) are capable of providing two‐photon excited luminescence (TPEL) and second harmonic generation (SHG) responses, much exceeding that of smaller Si NPs, which promises their use as probes for bi‐modal nonlinear optical bioimaging. It is finally demonstrated that the combination of TPEL and SHG channels makes possible efficient tracing of both separated Si NPs and their aggregations in different cell compartments, while the resolution of such an approach is enough to obtain 3D images. The obtained bi‐modal contrast provides lacking imaging functionality for large Si NPs and promises the development of novel cancer theranostic modalities on their basis.

In vivo optical bioimaging by using Nd-doped LaF3 luminescent nanorods in the second near-infrared window

Fluorescent imaging in the second near-infrared (NIR-II, 1000–1700 nm) region has attracted enormous research interest due to its advantages of unprecedentedly improved imaging sensitivity and spatial resolution. Herein, high quality NIR-II emissive LaF3:Nd3+ nanorods (NRs) with hexagonal phase structure and controlled size were synthesized by a hydrothermal method. And the Nd3+ doping induced NIR-II luminescent intensity evolution was studied. In vivo NIR-II bioimaging and real-time tracking reveal that these LaF3:Nd3+ NRs are mainly accumulated in liver and spleen. Moreover, in vivo toxicity assessment demonstrates that LaF3:Nd3+ NRs feature low biotoxicity and good biocompatibility in living animals. Therefore, the NIR-II emitting hexagonal phase LaF3 NRs with controlled size are promising probes for optical bioimaging.

Enabling magnetic resonance imaging of hollow-core microstructured optical fibers via nanocomposite coating.

Optical fibers are widely used in bioimaging systems as flexible endoscopes that are capable of low-invasive penetration inside hollow tissue cavities. Here, we report on the technique that allows magnetic resonance imaging (MRI) of hollow-core microstructured fibers (HC-MFs), which paves the way for combing MRI and optical bioimaging. Our approach is based on layer-by-layer assembly of oppositely charged polyelectrolytes and magnetite nanoparticles on the inner core surface of HC-MFs. Incorporation of magnetite nanoparticles into polyelectrolyte layers renders HC-MFs visible for MRI and induces the red-shift in their transmission spectra. Specifically, the transmission shifts up to 60 nm have been revealed for the several-layers composite coating, along with the high-quality contrast of HC-MFs in MRI scans. Our results shed light on marrying fiber-based endoscopy with MRI to open novel possibilities for minimally invasive clinical diagnostics and surgical procedures in vivo.

A General In Situ Growth Strategy of Designing Theranostic NaLnF4@Cu2−xS Nanoplatform for In Vivo NIR‐II Optical Imaging Beyond 1500 nm and Photothermal Therapy

AbstractTheranostic nanoprobes with a combination of highly sensitive optical bioimaging and photothermal therapy (PTT) are considered advanced tools for improving the detection precision and the imaging‐guided hyperthermal therapy efficacy against tumor in the biomedical area. Compared with the traditional visible/first near‐infrared (NIR‐I, 650–900 nm) light‐emitting optical probe, a nanoprobe capable of generating the second near‐infrared (NIR‐II, 1000–1700 nm) emission is emerging as the next‐generation optical imaging technique with high‐sensitivity, and high spatial/time resolution owing to its remarkably reduced photon scattering losses. However, a multifunctional theranostic nanoplatform incorporated with the new advanced NIR‐II optical imaging and PTT has not yet been explored. Herein, a general strategy for designing theranostic nanoplatforms by integrating NIR‐II optical bioimaging with photothermal functions via in situ growth of Cu2−xS quantum dots on the lanthanide nanorods is demonstrated. The as‐prepared NaLnF4:Yb/Er@Cu2−xS hybrid nanoprobes with a core‐satellite structure present excellent NIR‐II emission centered at 1525 nm, highly stable photothermal effects and good biocompatibility. These designed theranostic nanoprobes are utilized for NIR‐II optical imaging, small tumor detection (5 mm in diameter), and PTT. More importantly, non‐invasive brain vessel visualization with high spatial resolution (44.2 µm) through scalp and skull without craniotomy is demonstrated. Therefore, these results pave the way to designing new multifunction theranostic nanoplatforms for highly sensitive NIR‐II optical‐guided tumor detection, non‐invasive blood vessel imaging, and PTT.

Cell Imaging Using Two-Photon Excited CdS Fluorescent Quantum Dots Working within the Biological Window.

In recent years, two-photon excited semiconductor quantum dots (QDs) have been the subject of intense investigation due to their long excitation wavelength which helps to achieve deeper penetration and higher image resolution in optical bioimaging. In this paper, water-soluble CdS QDs were synthesized using a hydrothermal method and applied to human liver hepatocellular carcinoma (HepG2) cells. The first-principles calculation suggested that the S-rich defected structure contributes to a narrower band gap compared to the pristine structure. The resulting fluorescence wavelength was significantly red shifted, which was attributed to the deep defect states emission. The large Stokes shifts (> 200 nm) of the QDs can eliminate the possible cross-talk between the excitation light and the emission light. Two-photon induced red fluorescence emission can avoid overlapping with the autofluorescence emission of biological samples. The uptake and cell viability measurements of the HepG2 cells showed a good biocompatibility and a low toxicity of CdS QDs. Two-photon excited scanning microscopy images revealed that the HepG2 cells incubated with CdS QDs emitted bright red upconversion fluorescence and the fluorescence brightness was 38.2 times of that of the control group. These results support CdS QDs as a good candidate for application in cellular imaging.

808 nm laser-triggered NIR-II emissive rare-earth nanoprobes for small tumor detection and blood vessel imaging

Optical bioimaging in second near-infrared window (NIR-II, 1000–1700 nm) has been emerged as an indispensable tool for highly sensitive disease detection. In this work, intense NIR-II emissive polyacrylic acid (PAA) modified NaLuF4: Gd/Nd nanorods (PAA-NRs) with pure hexagonal phase and uniform size were explored for high sensitivity in vivo NIR-II bioimaging and optical imaging-guided small tumor detection. The NIR-II emission of the NaLuF4: Gd host can be readily adjusted by doping Nd3+, making it promising emission centered at 1056 nm and 1328 nm with high photo-stability. The time-dependent in vivo tracking results validate that the PAA-NRs are mainly accumulated in the reticuloendothelial system (RES) and excreted through the hepatic pathway. In addition, NIR-II optical imaging-guided small tumor (down to 5 mm) diagnosis was successfully achieved. Remarkably, in vivo small blood vessel with high spatial resolution (~105 μm) was detected clearly. And the histological tests reveal that our designed hydrophilic NRs present negligible toxicity effects and good biocompatibility in living animals. Besides the NIR-II emission, the PAA-NRs also present X-ray absorption features for X-ray bioimaging. These findings demonstrate that the explored lanthanide-based NRs with controllable size, efficient NIR-II emission and decent biocompatibility are promising NIR-II contrast agents for future biomedical applications, such as, early diagnosis of small tumor, vascular related disease imaging and angiogenesis diagnosis.

Theranostic Carbon Dots with Innovative NIR-II Emission for in Vivo Renal-Excreted Optical Imaging and Photothermal Therapy.

Carbon dots (CDs) with low biotoxicity, high photostability, and well-controlled size are highly desirable imaging agents for optical bioimaging. However, most of the CDs triggered by ultraviolet/blue light present visible/first near-infrared emissions shorter than 820 nm, impairing their imaging applications in vivo by low penetration depth. Hence, developing novel CD-based materials with second near-infrared (NIR-II) emission located in 1000-1700 nm region is an urgent task. Here, a novel NIR-II-emitting CD-based nanoprobe triggered by 808 nm laser is developed. The designed CDs with 900-1200 nm luminescence possess high quantum yield (QY-0.4%) and high biocompatibility, which have proven to be effective probes for in vivo NIR-II bioimaging. Notably, nearly 65% CDs are excreted from mouse urine within 6 h, demonstrating the rapid renal clearance of CDs. Furthermore, the designed CDs also exhibit high photothermal efficiency (30.6%), making them ideal materials for thermal ablation of cancer. Our findings pave the way of designing a multifunctional CD-based theranostic platform for simultaneously integrating the advanced NIR-II bioimaging and photothermal therapy of cancer.

Organic-to-water dispersible Mn:ZnS–ZnS doped core–shell quantum dots: synthesis, characterization and their application towards optical bioimaging and a turn-off fluorosensor

Dot-in-dot core/shell Mn:ZnS/ZnS QDs as a good fluorescent agent for bioimaging and a turn-off fluorescent probe for detection of heavy metal ions.

Prussian blue-coated lanthanide-doped core/shell/shell nanocrystals for NIR-II image-guided photothermal therapy.

Lanthanide-doped nanoparticles have long been stereotyped for optical luminescence bioimaging. However, they are known to be unable to produce therapeutic abilities. Here, we describe a lanthanide-based theranostic agent, namely, prussian blue (PB)-coated NaErF4@NaYF4@NaNdF4 core/shell/shell nanocrystals encapsulated in a phospholipid PEG micelle (PEG-CSS@PB), which showed switched imaging and hyperthermia abilities under distinct near infrared (NIR) light activation. The erbium (Er3+)-enriched inner core nanocrystals (NaErF4) enabled the emission of tissue-penetrating luminescence (1525 nm) in the second biological window (NIR-II, 1000-1700 nm), which endowed high-resolution optical imaging of the blood vessels and tumors under ∼980 nm excitation. High neodymium (Nd3+) concentrations in the epitaxial outer NaNdF4 shell introduced maximum cross relaxation processes that converted the absorbed NIR light (∼808 nm) into heat at high efficiencies, thus providing abilities for photothermal therapy (PTT). Importantly, the coated Prussian blue (PB) increased light absorption by about 10-fold compared to the composite free of PB, thus entailing a high light-to-heat conversion efficiency of ∼50.5%. This commensurated with that of well-established gold nanorods. As a result, the PEG-CSS@PB nanoparticles with MTT-determined low toxicities resulted in ∼80% death of HeLa cells at a dose of 600 μg mL-1 under 808 nm laser irradiance (1 W cm-2) for 10 min. Moreover, utilizing the same light dose, a single PTT treatment in tumor-bearing BALB/c mice shrunk the tumor size by ∼12-fold compared to the tumors without treatment. Our results, here, constituted a solid step forward to entitle lanthanide-based nanoparticles as theranostic agents in nanomedicine studies.

Deferred Registration of Nanophosphor Photoluminescence As a Platform for Optical Bioimaging

Detection systems with deferred registration of luminescence signals are promising for performing complex tasks of imaging of biological objects due to their simplicity and low cost. In the present work, β‑NaYF4:Tm3+Yb3+/NaYF4 nanocrystals with anti-Stokes photoluminescence have been used in deferred registration systems. It has been shown that there is a significant time delay between the exciting laser pulse and luminescence signal, which makes it possible to use this class of nanoparticles in the creation of wide-field imaging systems with deferred registration. The possibility of using nanoparticles for detecting a photoluminescence signal in the second transparency window of biotissue has been demonstrated. This system can be based on the resonance excitation and detection of the photoluminescence signal of Yb3+ ions.

Graphene-based all-optical multi-parameter regulations for an ultrafast fiber laser.

Ultrafast lasers with tunable capabilities of pulse duration and spectrum have widespread applications in telecommunication, spectroscopy, and nonlinear optical bio-imaging. However, traditional mechanical and electrical tuning methods are still challenging for precise and stable controlling. Based on graphene's photo-thermal effect, we tune the bandwidths and wavelengths of chirped fiber Bragg gratings with flexible graphene-coating approaches. By inserting the fabricated devices into an ultrafast fiber laser cavity, durations and wavelengths of the generated pulses can be all-optically tuned with sensitivities of 470fs/mW and 2.9pm/mW, separately. Such an optical-controlled method provides a compact and precise way to regulate various laser properties.

Robust and Biocompatible Functionalization of ZnS Nanoparticles by Catechol-Bearing Poly(2-methyl-2-oxazoline)s.

Zinc sulfide (ZnS) nanoparticles (NPs) are particularly interesting materials for their electronic and luminescent properties. Unfortunately, their robust and stable functionalization and stabilization, especially in aqueous media, has represented a challenging and not yet completely accomplished task. In this work, we report the synthesis of colloidally stable, photoluminescent and biocompatible core-polymer shell ZnS and ZnS:Tb NPs by employing a water-in-oil miniemulsion (ME) process combined with surface functionalization via catechol-bearing poly-2-methyl-2-oxazoline (PMOXA) of various molar masses. The strong binding of catechol anchors to the metal cations of the ZnS surface, coupled with the high stability of PMOXA against chemical degradation, enable the formation of suspensions presenting excellent colloidal stability. This feature, combined with the assessed photoluminescence and biocompatibility, make these hybrid NPs suitable for optical bioimaging.

Advances in nanomaterial based optical biosensing and bioimaging of apoptosis via caspase-3 activity: a review.

Caspase-3 plays a vital role in intrinsic and extrinsic pathways of programed cell death and in cell proliferation. Its detection is an important tool for early detection of some cancers and apoptosis-related diseases, and for monitoring the efficacy of pharmaceuticals and of chemo- and radiotherapy of cancers. This review (with 72 references) summarizes nanomaterial based methods for signal amplification in optical methods for the determination of caspase-3 activity. Following an introduction into the field, a first large section covers optical assays, with subsections on luminescent and chemiluminescence, fluorometric (including FRET based), and colorimetric assays. Further section summarize methods for bioimaging of caspase-3. A concluding section covers current challenges and future perspectives. Graphical Abstract ᅟ.

Ligand-Mediated Photobrightening and Photodarkening of CdSe/ZnS Quantum Dot Ensembles

Variation of the steady-state optical properties of quantum dots (QDs) caused by photoinduced charge transfer is a widely known phenomenon which hinders QD applications requiring long-term stability of the optical signal, for example, bioimaging, single-photon sources, molecular tracking, and lasing. Intense light irradiation may provoke QD photodarkening, when their photoluminescence (PL) quantum yield (QY) is reduced, or photobrightening, when the QY increases and QD PL becomes brighter. In order to understand the nature of these processes and to determine the optimal operating conditions for QDs with different core sizes and shell thicknesses, we have systematically studied the photodarkening and photobrightening phenomena for different wavelengths and intensities of irradiation. The results have shown that changes in the core size only slightly alter the effect of light irradiation on the QD optical properties, whereas the irradiation wavelength and shell thickness are the major factors that determine...

Doping Method Determines Para- or Superparamagnetic Properties of Photostable and Surface-Modifiable Quantum Dots for Multimodal Bioimaging

Semiconductor quantum dots (QDs) are widely used for optical applications and bioimaging. In comparison to organic dyes used for fluorescent labeling, QDs exhibit very high photostability and can be further surface modified. Equipping QDs with magnetic properties (mQDs) makes it possible to combine fluorescence and magnetic resonance imaging analyses. For this purpose, we have prepared water-dispersible and magnetic CdTe/ZnS mQDs, whereby ferrous ions are selectively incorporated in either their cores or their shells. This study aims at understanding the differences in optical, structural, and magnetic properties between these core- and shell-doped mQDs. Field-dependent isothermal magnetic susceptibility measurements show that shell-doped mQDs exhibit paramagnetic and their core-doped equivalents superparamagnetic behavior near room temperature. Shell doping results in about 1.7 times higher photoluminescence quantum yields and 1.4 times higher doping efficiency than core doping. X-ray diffraction pattern...

Organic Soluble LaPO4:Eu3+ Nanorods: Sensitization of Surface Eu3+ Ions and Phase Transfer in Water

AbstractChloroform soluble luminescent undoped and europium doped LaPO4 nanoparticles (NPs) were synthesized by oleic acid mediated solvothermal technique at low temperature (150 °C). These NPs have nanorod (NR) shape as confirmed by Transmission electron microscopy (TEM) and monoclinic phase as confirmed by X‐ray diffraction (XRD) measurement. Fourier transform infrared (FTIR) studies shows that oleic acid is bound to the surface of NRs. The Photoluminscence (PL) spectroscopy suggests that the undoped LaPO4 emits violet blue light when excited by UV‐light due to the surface oleate groups. In the doped NRs, oleate group serves as a sensitizer for the surface Eu3+ ions. In order to increase the emission output of these NRs, 2‐theonoyltrifluoroacetonate (TTA) is used to sensitize the emission of surface Eu3+ ions. PL spectroscopy and lifetime measurements clearly show that sensitization takes place only on the surface of the NRs while the core Eu3+ ions remains unsenesitized. The emission output is increased 33 times as compared to the unsensitized NRs. Further, these NRs were transferred from organic to the aqueous phase by citric acid. The NPs thus obtained are redispersible in water with good colloidal stability. Here, also sensitization of surface europium ions occurs in aqueous solution by TTA ligand. These luminescent organic and water soluble NRs further strengthen the application in the fields of optical display, lighting and bioimaging.

Synthesis and Characterization of Monodisperse Core-shell Lanthanide Upconversion Nanoparticles NaYF4: Yb,Tm/SiO2

Lanthanide up-converting luminescent nanoparticles (UCNPs) are exciting and promising materials for optical bioimaging, biosensor and theranostic due to their unique and advantageous optical and chemical properties. The UCNPs absorb low energy near-infrared (NIR) light and emit high-energy shorter wavelength photons (visible light). Their unique features allow them to overcome various problems associated with conventional imaging probes such as photostability, lack of toxicity, and to provide versatility for creating nanoplatforms with both imaging and therapeutic modalities. This paper reports synthesis and characterization of core-shell structured of NaYF4:Yb,Tm/SiO2 microspheres. The synthesis of lanthanide upconversion nanoparticles NaYF4:Yb,Tm was prepared by thermal decomposition process which involves dissolving organic precursors in high-boiling-point solvents oleic acid (OA) and octadecene (ODE). After that, the NaYF4:Yb,Tm phosphors was coated by silica via reverse microemulsion process to obtain core-shell structured NaYF4:Yb,Tm/SiO2. Scanning electron microscopy, transmission electron microscopy, specific area electron diffraction, and photoluminescence were applied to characterize these samples. The obtained core-shell structured NaYF4:Yb,Tm/SiO2 phosphors exhibit a perfect cubic morphology with narrow size distribution and smooth surface. Upon IR excitation at 980 nm, the NaYF4:Yb,Tm/SiO2 samples exhibit whitish blue upconversion (UC) luminescence, respectively. These phosphors show potential applications in the displaying on biological fields and biosensing.