Nanomedicine and Early Cancer Diagnosis: Molecular Imaging using Fluorescence Nanoparticles

Incorporating nanotechnology into fluorescent imaging and magnetic resonance imaging (MRI) has shown promising potential for accurate diagnosis of cancer at an earlier stage than the conventional imaging modalities. Molecular imaging (MI) aims to quantitatively characterize, visualize, and measure the biological processes or living cells at molecular and genetic levels. MI modalities have been exploited in different applications including noninvasive determination and visualization of diseased tissues, cell trafficking visualization, early detection, treatment response monitoring, and in vivo visualization of living cells. High-affinity molecular probe and imaging modality to detect the probe are the two main requirements of MI. Recent advances in nanotechnology and allied modalities have facilitated the use of nanoparticles (NPs) as MI probes. Within the extensive group of NPs, fluorescent NPs play a prominent role in optical molecular imaging. The fluorescent NPs used in molecular and cellular imaging can be categorized into three main groups including quantum dots (QDs), upconversion, and dyedoped NPs. Fluorescent NPs have great potential in targeted theranostics including cancer imaging, immunoassay- based cells, proteins and bacteria detections, imaging-guided surgery, and therapy. Fluorescent NPs have shown promising potentials for drug and gene delivery, detection of the chromosomal abnormalities, labeling of DNA, and visualizing DNA replication dynamics. Multifunctional NPs have been successfully used in a single theranostic modality integrating diagnosis and therapy. The unique characteristics of multifunctional NPs make them potential theranostic agents that can be utilized concurrently for diagnosis and therapy. This review provides the state of the art of the applications of nanotechnologies in early cancer diagnosis focusing on fluorescent NPs, their synthesis methods, and perspectives in clinical theranostics.

Oleogel-mediated transdermal delivery of white emitting NaYF4 conjugated with Rose Bengal for the generation of reactive oxygen species through NIR-upconversion

The transdermal route for the delivery of therapeutic agents to the inner skin tissues for non-invasive photodynamic therapy; though constitutes a desired modality for treating skin cancer, the success has been limited due to the insurmountable nature of the stratum corneum (SC). In this context, for the first time we report the localization of photosensitizer-conjugated upconversion (UC) particles to the deeper dermal region by overcoming SC through an oleogel-mediated transport mechanism for NIR-induced photodynamic production of reactive oxygen species (ROS). We developed soybean oil and stearic acid based oleogels by incorporating photoluminescent white light emitting NaYF₄ (WEN) upconversion (UC) particles conjugated with Rose Bengal (RB), termed as WEN-RB-G. Similarly, we fabricated another type of oleogel by incorporating Li⁺ doped WEN based UC particles (RB conjugated), with 10 times more photoluminescence intensity, termed as LiWEN-RB-G. Based on the skin permeation enhancing effect of the constituents of the oleogels, we demonstrated the permeation of these two types of conjugated particles in microgram scale through the full thickness of the pig ear skin model within 48 h. The localization of the conjugated particles throughout the skin tissue including dermal and epidermal region was confirmed by confocal microscopy. We also conducted a comparative assessment on WEN-RB-G and LiWEN-RB-G for the suitability of ROS generation and bioimaging under NIR activation. The 'proof of principle' concept reported here is expected to frame a gateway in future for NIR-induced photo-theranostics targeting skin cancer.

Recent advances of lanthanide-doped upconversion nanoparticles for biological applications

Near infrared (NIR) excited lanthanide-doped upconversion nanoparticles (UCNPs) are emerging as a new type of fluorescent tag for biological applications, which can emit multi-photon ultraviolet, visible or NIR luminescence for imaging or activation of photosensitive molecules. Here, we present a comprehensive review on recent advances of UCNPs for a manifold of biological applications, including upconversion mechanisms, building bright multicolor upconversion nanocrystals, single nanoparticle and super resolution imaging, in vivo optical and multimodal imaging, photodynamic therapy, light-controlled drug release, biosensing, and toxicities. Our perspectives on the future development of UCNPs are also described.

Design of novel lanthanide-doped core-shell nanocrystals with dual up-conversion and down-conversion luminescence for anti-counterfeiting printing

Development of advanced luminescent nanomaterials and technologies is of great significance for anti-counterfeiting applications in global economy, security, and human health, but has proved to be a great challenge. In this work, we design, synthesize, and characterize mono-disperse, dumbbell-shaped lanthanide-doped NaYF₄@NaGdF₄ core-shell nanoparticles (CSNPs) with dual-mode fluorescence by coating the NaGdF₄:Ln'³⁺ shell onto NaYF₄:Ln³⁺ core nanospheres via a two-step oleic acid mediated thermal decomposition process. Different from the conventional synthesis method to produce spherical nanoparticles, the epitaxial growth of the NaGdF₄:Ln'³⁺ shell onto the nanosphere cores and the lattice mismatch between β-NaGdF₄ and β-NaYF₄ nanocrystals enable the formation of dumbbell-shaped CSNPs, as evidenced by the morphological evolution of CSNPs and as explained by the Ostwald ripening growth mechanism. By tailoring different doped lanthanide ions in the core and the shell, the resultant CSNPs exhibit tunable but different up-/down-conversion luminescence under the irradiation of a 980 nm laser and 254 nm UV light, respectively. Finally, these hydrophilic CSNPs are further fabricated into environmentally benign luminescent inks for inkjet printing to create a variety of dual-mode fluorescent patterns (peacock, temple, and a logo of "Hunan University of Technology") on different paper-based substrates (A4 paper, envelope, and postcard). Our dual-mode light-responsive CSNPs, along with an easy fabrication method, provide a simple and promising material and technique for anti-counterfeiting applications.

Lanthanide Doped Near Infrared Active Upconversion Nanophosphors: Fundamental Concepts, Synthesis Strategies, and Technological Applications

Near infrared (NIR) light utilization in a range of current technologies has gained huge significance due to its abundance in nature and nondestructive properties. NIR active lanthanide (Ln) doped upconversion nanomaterials synthesized in controlled shape, size, and surface functionality can be combined with various pertinent materials for extensive applications in diverse fields. Upconversion nanophosphors (UCNP) possess unique abilities, such as deep tissue penetration, enhanced photostability, low toxicity, sharp emission peaks, long anti-Stokes shift, etc., which have bestowed them with prodigious advantages over other conventional luminescent materials. As new generation fluorophores, UCNP have found a wide range of applications in various fields. In this Review, a comprehensive overview of lanthanide doped NIR active UCNP is provided by discussing the fundamental concepts including the different mechanisms proposed for explaining the upconversion processes, followed by the different strategies employed for the synthesis of these materials, and finally the technological applications of UCNP, mainly in the fields of bioimaging, drug delivery, sensing, and photocatalysis by highlighting the recent works in these areas. In addition, a brief note on the applications of UCNP in other fields is also provided along with the summary and future perspectives of these materials.

Three primary color emissions from single multilayered nanocrystals

The achievement of three-primary-color luminescence in a single material will lead to revolutionary developments of many advanced applications such as dynamic display with ultra-high resolution, and complex anti-counterfeiting. Here we report the realization of steady-state three-primary-color emission in single multilayered NaYF4 upconversion (UC) nanoparticles. In this core-shell structure, a novel design of a tri-sensitizer, i.e., Nd3+, Yb3+ and Er3+ ions, is utilized, which effectively absorbs the excitation photons of 808, 980 and 1550 nm, and then exhibits blue, red and green emissions, respectively. By simply combining the three primary color emissions, tunable full-color luminescence was achieved in this single material. These nanoparticles have demonstrated promising potential applications in dynamic display and multiple anti-counterfeiting.

Distinct mechanisms for the upconversion of NaYF4:Yb3+,Er3+ nanoparticles revealed by stimulated emission depletion

Upconversion nanoparticles (UCNPs) have attracted enormous interest over the past few years because of their unique optical properties and potential for use in various applications such as bioimaging probes, biosensors, and light-harvesting materials for photovoltaics. The improvement of imaging resolution is one of the most important goals for UCNPs used in biological applications. Super-resolution imaging techniques that overcome the fundamental diffraction limit of light rely on the photochemistry of organic dyes or fluorescent proteins. Here we report our progress toward super-resolution microscopy with UCNPs. We found that the red emission (655 nm) of core/shell UCNPs with the structure NaYF₄:Yb³⁺,Er³⁺/NaYF₄ could be modulated by emission depletion (ED) of the intermediate state that interacts resonantly with an infrared beam (1540 nm). In contrast, the green emission bands (525 and 545 nm) of the UCNPs were less affected by irradiation with the infrared beam. The origin of such distinct behaviors between the green and red emissions was attributed to their different photophysical pathways.

Molecular design of upconversion nanoparticles for gene delivery

Due to their large anti-Stokes shifts, sharp emission spectra and long excited-state lifetimes, upconversion nanoparticles (UCNPs) have attracted an increasing amount of research interests, and have shown great potential for enhancing the practical utility of gene therapy, whose versatility has been limited by existing gene delivery technologies that are basically mono-functional in nature. Despite this, up to now in-depth analysis of the development of UCNPs for gene delivery has been scant in the literature, even though there has been an upsurge of reviews on the chemistry of UCNPs and their applications in bioimaging and drug delivery. To fill this gap, this review aims to present the latest advances in the development and applications of UCNPs as gene carriers. Prior to describing the prominent works published in the field, a critical view on the properties, chemistry and molecular design of UCNPs for gene delivery is provided. With a synopsis of the recent advances in UCNP-mediated gene delivery, challenges and opportunities could be illuminated for clinical translation of works in this nascent field of research.

Advanced Functional Nanomaterials for Theranostics

Nanoscale materials have been explored extensively as agents for therapeutic and diagnostic (i.e. theranostic) applications. Research efforts have shifted from exploring new materials in vitro to designing materials that function in more relevant animal disease models, thereby increasing potential for clinical translation. Current interests include non-invasive imaging of diseases, biomarkers and targeted delivery of therapeutic drugs. Here, we discuss some general design considerations of advanced theranostic materials and challenges of their use, from both diagnostic and therapeutic perspectives. Common classes of nanoscale biomaterials, including magnetic nanoparticles, quantum dots, upconversion nanoparticles, mesoporous silica nanoparticles, carbon-based nanoparticles and organic dye-based nanoparticles, have demonstrated potential for both diagnosis and therapy. Variations such as size control and surface modifications can modulate biocompatibility and interactions with target tissues. The needs for improved disease detection and enhanced chemotherapeutic treatments, together with realistic considerations for clinically translatable nanomaterials will be key driving factors for theranostic agent research in the near future.

Tuning the morphology, luminescence and magnetic properties of hexagonal-phase NaGdF4: Yb, Er nanocrystals via altering the addition sequence of the precursors

Hexagonal-phase NaGdF₄: Yb, Er upconversion nanocrystals (UCNCs) with tunable morphology and properties were successfully prepared via a thermal decomposition method. The influences of the adding sequence of the precursors on the morphology, chemical composition, luminescence and magnetic properties were investigated by transmission electron microscopy (TEM), inductively coupled plasma-atomic emission spectrometry (ICP-AES), upconversion (UC) spectroscopy, and a vibrating sample magnetometer (VSM). It was found that the resulting nanocrystals, with different sizes ranging from 24 to 224 nm, are in the shape of spheres,  hexagonal plates and flakes; moreover, the composition percentage of Yb³⁺-Er³⁺ and Gd³⁺ ions was found to vary in a regular pattern with the adding sequence. Furthermore, the intensity ratios of emission colors (f _g/r, f _g/p), and the magnetic mass susceptibility of hexagonal-phase NaGdF₄: Yb, Er nanocrystals change along with the composition of the nanocrystals. A positive correlation between the susceptibility and f _g/r of NaGdF₄: Yb, Er was proposed. The decomposition processes of the precursors were investigated by a thermogravimetric (TG) analyzer. The result indicated that the decomposition of the resolved lanthanide trifluoroacetate is greatly different from lanthanide trifluoroacetate powder. It is of tremendous help to recognize the decomposition process of the precursors and to understand the related reaction mechanism.

Simple method for simultaneously achieving red and green up-conversion luminescence

The simultaneous emission of red and green light with high brightness and color purity was obtained from Er³⁺-doped NaYbF₄-based up-conversion nanoparticles excited by 980 and 1550 nm.

Low-temperature molten-salt synthesis and upconversion of novel hexagonal NaBiF4:Er3+/Yb3+ micro-/nanocrystals

A series of NaBiF₄:Er³⁺/Yb³⁺ micro-/nanocrystals were synthesized via the low-temperature molten-salt method in NH₄NO₃ flux.

The preferred upconversion pathway for the red emission of lanthanide-doped upconverting nanoparticles, NaYF4:Yb(3+),Er(3.)

Lanthanide-doped upconverting nanoparticles (UCNPs, NaYF4:Yb(3+),Er(3+)) are well known for emitting visible photons upon absorption of two or more near-infrared (NIR) photons through energy transfer from the sensitizer (Yb(3+)) to the activator (Er(3+)). Of the visible emission bands (two green and one red band), it has been suggested that the red emission results from two competing upconversion pathways where the non-radiative relaxation occurs after the second energy transfer (pathway A, (4)I15/2 → (4)I11/2 → (4)F7/2 → (2)H11/2 → (4)S3/2 → (4)F9/2 → (4)I15/2) or between the first and the second energy transfer (pathway B, (4)I15/2 → (4)I11/2 → (4)I13/2 → (4)F9/2 → (4)I15/2). However, there has been no clear evidence or thorough analysis of the partitioning between the two pathways. We examined the spectra, power dependence, and time profiles of UCNP emission at either 980 nm or 488 nm excitation, to address which pathway is preferred. It turned out that the pathway B is predominant for the red emission over a wide range of excitation powers.

Synthesis and characterization of upconversion nanoparticles with shell structure and ligand-free hydrophilic modification

Rare-earth upconversion nanoparticles (UCNPs) with α and β phases were prepared. UCNPs with core–shell structure were prepared and modified to be hydrophilic by ligand-free hydrophilic modification.

Lanthanide-doped upconversion nano-bioprobes: electronic structures, optical properties, and biodetection

The latest advances in lanthanide-doped upconversion nanoparticles were comprehensively reviewed, which covers from their fundamental photophysics to biodetection.

NaYF4:Yb,Er–MoS2: from synthesis and surface ligand stripping to negative infrared photoresponse

The synthesis, surface ligand stripping (SOCl₂/DMF treatment), and unusual negative infrared photoresponse of new NaYF₄:Yb,Er–MoS₂ sheet nanocomposites were reported.

Synergetically enhanced near-infrared photoresponse of reduced graphene oxide by upconversion and gold plasmon

A new route to improve responsivity of reduced graphene oxide (rGO)-based near-infrared photodetectors is reported by coupling upconversion and gold plasmon. Near-infrared light is converted by upconversion nanoparticle into shorter wavelengths that can readily be absorbed by rGO. Further coupling of plasmonic layer increased upconversion emissions and rGO absorption, resulting in an overall enhancement of photo-responsivity by 10 times.

3-Dimensional photonic crystal surface enhanced upconversion emission for improved near-infrared photoresponse

The enhancement of upconversion luminescence of lanthanide-ion doped fluoride upconversion nanoparticles (UCNPs) is particularly important and highly required for their myriad applications in sensing, photoelectronic devices and bio-imaging. In this work, the amplification of luminescence in NaYF4:Yb/Er and NaYF4:Yb/Tm UCNPs in close proximity to the three-dimensional photonic crystal (3D PC) surface for improved near-infrared photoresponse of a carbon nanotube-based phototransistor is reported. The self-assembled opal 3D PCs with polystyrene sphere sizes of 200, 290 and 360 nm that exhibit reflection peaks of 450, 650 and 800 nm respectively were used for upconversion enhancement, and around 30 times enhancement was obtained for NaYF4:Yb/Er and NaYF4:Yb/Tm UCNPs. Time-resolved upconversion emission and 3D PC transmittance-dependent upconversion enhancement reveal that the enhanced absorption and the extraction effects, resulting from the enhanced non-resonant pump excitation field and the strong coherent scattering provided by 3D PCs respectively, are responsible for the large enhancement. As a proof-of-concept experiment, the prepared 3D PC/NaYF4:Yb/Tm UCNP coupled material layer was introduced into the carbon nanotube-based phototransistor. It is shown that the photoresponsivity of the device to near-infrared light was improved by 10 times with respect to the control device with carbon nanotubes only, which reveals the promising applications of this coupled material in photoelectronic devices such as photovoltaics and other types of phototransistors.

Multicolor tunability and upconversion enhancement of fluoride nanoparticles by oxygen dopant

The ability to manipulate the upconversion luminescence of lanthanide-ion doped fluoride upconversion nanoparticles (UCNPs) is particularly important and highly desired due to their wide applications in color displays, multiplexing bioassays and multicolor imaging. Here, we developed a strategy for simultaneously tuning color output and enhancing upconversion emission of Yb/Er doped fluoride UCNPs, based on adjusting the oxygen doping level. The synthesis of multicolored multifunctional NaGdF4:Yb,Er UCNPs was used as the model host system to demonstrate this protocol. Ammonium nitrate (NH4NO3) was used as the oxygen source and added into the reaction system at the beginning stage of nucleation and growth process of fluoride UCNPs, which facilitates the formation of enough oxygen atoms and the diffusion of these into the fluoride host matrix. The results revealed that multicolour output and upconversion enhancement mainly resulted from the variation of phonon energy and crystal field symmetry of the host lattice, respectively. This strategy can be further expanded to other fluoride host matrices. As an example of an application, multicolored UCNPs were used as a color converter in light emitting diodes, which can effectively convert near-infrared light into visible light. It is expected that these multicolored UCNPs will be promising for applications in multiplexing biodetection, bioimaging (optical and magnetic resonance imaging) and other optical technologies, and the present method for the control of O(2-) doping may also be used in other functional nanomaterials.

Fluorescence in nanobiotechnology: sophisticated fluorophores for novel applications

Nanobiotechnology is one of the fastest growing and broadest-ranged interdisciplinary subfields of the nanosciences. Countless hybrid bio-inorganic composites are currently being pursued for various uses, including sensors for medical and diagnostic applications, light- and energy-harvesting devices, along with multifunctional architectures for electronics and advanced drug-delivery. Although many disparate biological and nanoscale materials will ultimately be utilized as the functional building blocks to create these devices, a common element found among a large proportion is that they exert or interact with light. Clearly continuing development will rely heavily on incorporating many different types of fluorophores into these composite materials. This review covers the growing utility of different classes of fluorophores in nanobiotechnology, from both a photophysical and a chemical perspective. For each major structural or functional class of fluorescent probe, several representative applications are provided, and the necessary technological background for acquiring the desired nano-bioanalytical information are presented.

Combinatorial discovery of lanthanide-doped nanocrystals with spectrally pure upconverted emission

Nanoparticles doped with lanthanide ions exhibit stable and visible luminescence under near-infrared excitation via a process known as upconversion, enabling long-duration, low-background biological imaging. However, the complex, overlapping emission spectra of lanthanide ions can hinder the quantitative imaging of samples labeled with multiple upconverting probes. Here, we use combinatorial screening of multiply doped NaYF(4) nanocrystals to identify a series of doubly and triply doped upconverting nanoparticles that exhibit narrow, spectrally pure emission spectra at various visible wavelengths. We then developed a comprehensive kinetic model validated by our extensive experimental data set. Applying this model, we elucidated the energy transfer mechanisms giving rise to spectrally pure emission. These mechanisms suggest design rules for electronic level structures that yield robust color tuning in lanthanide-doped upconverting nanoparticles. The resulting materials will be useful for background-free multicolor imaging and tracking of biological processes.

Recent advances in synthesis and surface modification of lanthanide-doped upconversion nanoparticles for biomedical applications

Lanthanide (Ln)-doped upconversion nanoparticles (UCNPs) with appropriate surface modification can be used for a wide range of biomedical applications such as bio-detection, cancer therapy, bio-labeling, fluorescence imaging, magnetic resonance imaging and drug delivery. The upconversion phenomenon exhibited by Ln-doped UCNPs renders them tremendous advantages in biological applications over other types of fluorescent materials (e.g., organic dyes, fluorescent proteins, gold nanoparticles, quantum dots, and luminescent transition metal complexes) for: (i) enhanced tissue penetration depths achieved by near-infrared (NIR) excitation; (ii) improved stability against photobleaching, photoblinking and photochemical degradation; (iii) non-photodamaging to DNA/RNA due to lower excitation light energy; (iv) lower cytotoxicity; and (v) higher detection sensitivity. Ln-doped UCNPs are therefore attracting increasing attentions in recent years. In this review, we present recent advances in the synthesis of Ln-doped UCNPs and their surface modification, as well as their emerging applications in biomedicine. The future prospects of Ln-doped UCNPs for biomedical applications are also discussed.

Upconversion nanomaterials: synthesis, mechanism, and applications in sensing

Upconversion is an optical process that involves the conversion of lower-energy photons into higher-energy photons. It has been extensively studied since mid-1960s and widely applied in optical devices. Over the past decade, high-quality rare earth-doped upconversion nanoparticles have been successfully synthesized with the rapid development of nanotechnology and are becoming more prominent in biological sciences. The synthesis methods are usually phase-based processes, such as thermal decomposition, hydrothermal reaction, and ionic liquids-based synthesis. The main difference between upconversion nanoparticles and other nanomaterials is that they can emit visible light under near infrared irradiation. The near infrared irradiation leads to low autofluorescence, less scattering and absorption, and deep penetration in biological samples. In this review, the synthesis of upconversion nanoparticles and the mechanisms of upconversion process will be discussed, followed by their applications in different areas, especially in the biological field for biosensing.