Conjugation of transferrin to azide-modified CdSe/ZnS core-shell quantum dots using cyclooctyne click chemistry

Click Bioconjugation - Modifying Proteins using Click-Like Chemistry

Bioconjugation is dealing with the chemical modification of proteins. The reactions used exploit either the intrinsic chemical reactivity of the biomolecule or introduce functionalities that can then be subsequently reacted without interfering with other functional groups of the biological entity. Perfectly selective, high yielding chemical transformations are needed that can be run in aqueous environment at mild pH conditions. Requirements that have an obvious overlap with the definition of click chemistry. This review shows a selection of successfully applied click-type reactions in bioconjugation as well as some recent developments to broaden the chemical toolbox to meet the challenge of a selective, bioorthogonal modification of biomolecules.

Onco-Receptors Targeting in Lung Cancer via Application of Surface-Modified and Hybrid Nanoparticles: A Cross-Disciplinary Review

Lung cancer is among the most prevalent and leading causes of death worldwide. The major reason for high mortality is the late diagnosis of the disease, and in most cases, lung cancer is diagnosed at fourth stage in which the cancer has metastasized to almost all vital organs. The other reason for higher mortality is the uptake of the chemotherapeutic agents by the healthy cells, which in turn increases the chances of cytotoxicity to the healthy body cells. The complex pathophysiology of lung cancer provides various pathways to target the cancerous cells. In this regard, upregulated onco-receptors on the cell surface of tumor including epidermal growth factor receptor (EGFR), integrins, transferrin receptor (TFR), folate receptor (FR), cluster of differentiation 44 (CD44) receptor, etc. could be exploited for the inhibition of pathways and tumor-specific drug targeting. Further, cancer borne immunological targets like T-lymphocytes, myeloid-derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs), and dendritic cells could serve as a target site to modulate tumor activity through targeting various surface-expressed receptors or interfering with immune cell-specific pathways. Hence, novel approaches are required for both the diagnosis and treatment of lung cancers. In this context, several researchers have employed various targeted delivery approaches to overcome the problems allied with the conventional diagnosis of and therapy methods used against lung cancer. Nanoparticles are cell nonspecific in biological systems, and may cause unwanted deleterious effects in the body. Therefore, nanodrug delivery systems (NDDSs) need further advancement to overcome the problem of toxicity in the treatment of lung cancer. Moreover, the route of nanomedicines’ delivery to lungs plays a vital role in localizing the drug concentration to target the lung cancer. Surface-modified nanoparticles and hybrid nanoparticles have a wide range of applications in the field of theranostics. This cross-disciplinary review summarizes the current knowledge of the pathways implicated in the different classes of lung cancer with an emphasis on the clinical implications of the increasing number of actionable molecular targets. Furthermore, it focuses specifically on the significance and emerging role of surface functionalized and hybrid nanomaterials as drug delivery systems through citing recent examples targeted at lung cancer treatment.

Nanoparticles Targeting Receptors on Breast Cancer for Efficient Delivery of Chemotherapeutics

The journey of chemotherapeutic drugs from the site of administration to the site of action is confronted by several factors including low bioavailability, uneven distribution in major organs, limited accessibility of drug molecules to the distant tumor tissues, and lower therapeutic indexes. These unavoidable features of classical chemotherapeutics necessitate an additional high, repetitive dose of drugs to obtain maximum therapeutic responses with the result of unintended adverse side effects. An erratic tumor microenvironment, notable drawbacks of conventional chemotherapy, and multidrug-resistant mechanisms of breast cancer cells warrant precisely designed therapeutics for the treatment of cancers. In recent decades, nanoparticles have been deployed for the delivery of standard anticancer drugs to maximize the therapeutic potency while minimizing the adverse effects to increase the quality and span of life. Several organic and inorganic nanoplatforms that have been designed exploiting the distinctive features of the tumor microenvironment and tumor cells offer favorable physicochemical properties and pharmacokinetic profiles of a parent drug, with delivery of higher amounts of the drug to the pathological site and its controlled release, thereby improving the balance between its efficacy and toxicity. Advances to this front have included design and construction of targeted nanoparticles by conjugating homing devices like peptide, ligand, and Fab on the surface of nanomaterials to navigate nanoparticledrug complexes towards the target tumor cell with minimal destruction of healthy cells. Furthermore, actively targeting nanoparticles can facilitate the delivery and cellular uptake of nanoparticle-loaded drug constructs via binding with specific receptors expressed aberrantly on the surface of a tumor cell. Herein, we present an overview of the principle of targeted delivery approaches, exploiting drug-nanoparticle conjugates with multiple targeting moieties to target specific receptors of breast cancer cells and highlighting therapeutic evaluation in preclinical studies. We conclude that an understanding of the translational gap and challenges would show the possible future directions to foster the development of novel targeted nanotherapeutics.

Novel Method of Clickable Quantum Dot Construction for Bioorthogonal Labeling

Bioorthogonal chemistry has been considered as a powerful tool for biomolecule labeling due to its site specificity, moderate reaction conditions, high yield, and simple post-treatment. Covalent coupling is commonly used to modify quantum dots (QDs) with bioorthogonal functional group (azide or cycloalkyne), but it has a negative effect in the decrease of QDs' quantum yield and stability and increase of QDs' hydrodynamic diameter. To overcome these disadvantages, we propose a novel method for the preparation of two kinds of clickable QDs by the strong interaction of -SH with metal ions. One system involves azide-DNA-functionalized QDs, which are used for bioconjugation with dibenzocyclooctyne (DBCO)-modified glucose oxidase (GOx) to form a GOx-QDs complex. After bioconjugation, the stability of QDs was improved, and the activity of GOx was also enhanced. The GOx-QDs complex was used for rapid detection of blood glucose by spectroscopy, naked eye, and paper-based analytical devices. The second system involves DBCO-DNA-functionalized QDs, which are used for an in situ bioorthogonal labeling of HeLa cells through metabolic oligosaccharide engineering. Therefore, these clickable QDs based on DNA functionalization can be applied for rapid and effective labeling of biomolecules of interest.

Enzyme- or light-triggered cyclopropenes for bioorthogonal ligation

Since first reported at the beginning of the 21st century, bioorthogonal reactions have become powerful tools for investigating biological systems. Here, we review several classic and current bioorthogonal reactions, including the Staudinger-Bertozzi ligation, strain-promoted azide-alkyne cycloaddition (SPAAC), 1,3-dipolar cycloaddition, and tetrazine-alkene ligation. We discuss the capabilities and limitations of the subset of current bioorthogonal reactions that can be "turned on" by exposure to light or an enzyme. Finally, we focus on our recently developed turn-on cyclopropenes, which can be activated for reaction with tetrazines by exposure to light or enzymes, like nitroreductase, depending on the modular reaction caging group appended to the cyclopropene. We discuss the caged cyclopropene's molecular design and synthesis, and we discuss experiments to evaluate and verify reactivity both in vitro and in vivo.

Encapsulating Quantum Dots in Lipid-PEG Micelles and Subsequent Copper-Free Click Chemistry Bioconjugation

The utility of quantum dots (QDs) for biological applications is predicated on stably dispersing the particles in aqueous media. During transfer from apolar organic solvents to water, the optical properties of the fluorescent nanoparticles must be maintained; additionally, the resulting colloid should be monodisperse and stable against aggregation. Furthermore, the hydrophilic coating should confer functional groups or conjugation handles to the QDs, as biofunctionalization is often critical to biosensing and bioimaging applications. Micelle encapsulation is an excellent technique for conferring hydrophilicity and conjugation handles to QDs. One interesting conjugation handle that can easily be added to the QDs is an azide group, which conjugates to strained alkynes via strain promoted azide-alkyne cycloaddition (SPAAC) reactions. SPAAC, or copper-free click chemistry, utilizes very mild reaction conditions, involves reactive groups that are bio-orthogonal, and is nearly quantitative. Micelle encapsulation is also very mild and preserves the optical properties of the QDs nearly perfectly. The combination of these approaches comprises a mild, effective, and straightforward approach to preparing functionalized QDs for biological applications.

Quantum dots in biomedical applications

Semiconducting nanoparticles, more commonly known as quantum dots, possess unique size and shape dependent optoelectronic properties. In recent years, these unique properties have attracted much attention in the biomedical field to enable real-time tissue imaging (bioimaging), diagnostics, single molecule probes, and drug delivery, among many other areas. The optical properties of quantum dots can be tuned by size and composition, and their high brightness, resistance to photobleaching, multiplexing capacity, and high surface-to-volume ratio make them excellent candidates for intracellular tracking, diagnostics, in vivo imaging, and therapeutic delivery. We discuss recent advances and challenges in the molecular design of quantum dots are discussed, along with applications of quantum dots as drug delivery vehicles, theranostic agents, single molecule probes, and real-time in vivo deep tissue imaging agents. We present a detailed discussion of the biodistribution and toxicity of quantum dots, and highlight recent advances to improve long-term stability in biological buffers, increase quantum yield following bioconjugation, and improve clearance from the body. Last, we present an outlook on future challenges and strategies to further advance translation to clinical application. STATEMENT OF SIGNIFICANCE: Semiconducting nanoparticles, commonly known as quantum dots, possess unique size and shape dependent electrical and optical properties. In recent years, they have attracted much attention in biomedical imaging to enable diagnostics, single molecule probes, and real-time imaging of tumors. This review discusses recent advances and challenges in the design of quantum dots, and highlights how these strategies can further advance translation to clinical applications.

Active Targeting Strategies Using Biological Ligands for Nanoparticle Drug Delivery Systems

Targeting nanoparticle (NP) carriers to sites of disease is critical for their successful use as drug delivery systems. Along with optimization of physicochemical properties, researchers have focused on surface modification of NPs with biological ligands. Such ligands can bind specific receptors on the surface of target cells. Furthermore, biological ligands can facilitate uptake of modified NPs, which is referred to as 'active targeting' of NPs. In this review, we discuss recent applications of biological ligands including proteins, polysaccharides, aptamers, peptides, and small molecules for NP-mediated drug delivery. We prioritized studies that have demonstrated targeting in animals over in vitro studies. We expect that this review will assist biomedical researchers working with NPs for drug delivery and imaging.

Engineering Antibodies with C-Terminal Sortase-Mediated Modification for Targeted Nanomedicine

The current advances in nanoengineered materials coupled with the precise targeting capability of recombinant antibodies can create nanoscale diagnostics and therapeutics which show enhanced accumulation and extended retention at a target tissue. Smaller antibodies such as single-chain variable fragments (scFv) preserve the selective and strong binding of their parent antibody to their antigen with the benefits of low immunogenicity, more efficient tissue penetration and easy introduction of functional residues suitable for site-specific conjugation. This is of high importance as nonspecific antibody modification often involves attachment to free cysteine or lysine amino acids which may reside in the active site, leading to reduced antigen binding.In this chapter, we outline a facile and versatile chemoenzymatic approach for production of targeted nanocarrier scFv conjugates using the bacterial trans-peptidase Sortase A (Srt A). Srt A efficiently mediates sequence-specific peptide ligation under mild conditions and has few undesirable side reactions. We first describe the production, purification and characterization of Srt A enzyme and a scFv construct which targets activated platelets, called scFv_{anti-GPIIb/IIIa}. Following this, our protocol illustrates the chemoenzymatic modification of the antibody at the C-terminus with an orthogonal click chemistry linker. This avoids any random attachment to the biologically active antigen binding site of the antibody. Finally, we describe the modification of a nanoparticle surface with scFv attachment via two methods: (1) direct Sortase-mediated conjugation; or (2) a two-step system which consists of scFv Sortase-mediated conjugation followed by strain promoted azide-alkyne cycloaddition. Finally, methodology is described to assess the successful assembly of targeted particles.

Fluorescent squaramide ligands for cellular imaging and their encapsulation in cubosomes

Two new fluorescent squaramides bearing quinoline (L1) and naphthalene (L2) as fluorogenic fragments were synthesized and investigated as possible cellular imaging probes as free molecules and when loaded in monoolein-based cubosomes.

Transferrin receptors-targeting nanocarriers for efficient targeted delivery and transcytosis of drugs into the brain tumors: a review of recent advancements and emerging trends

Treatment of glioblastoma multiforme (GBM) is a predominant challenge in chemotherapy due to the existence of blood-brain barrier (BBB) which restricts delivery of chemotherapeutic agents to the brain together with the problem of drug penetration through hard parenchyma of the GBM. With the structural and mechanistic elucidation of the BBB under both physiological and pathological conditions, it is now viable to target central nervous system (CNS) disorders utilizing the presence of transferrin (Tf) receptors (TfRs). However, overexpression of these TfRs on the GBM cell surface can also help to avoid restrictions of GBM cells to deliver chemotherapeutic agents within the tumor. Therefore, targeting of TfR-mediated delivery could counteract drug delivery issues in GBM and create a delivery system that could cross the BBB effectively to utilize ligand-conjugated drug complexes through receptor-mediated transcytosis. Hence, approach towards successful delivery of antitumor agents to the gliomas has been making possible through targeting these overexpressed TfRs within the CNS and glioma cells. This review article presents a thorough analysis of current understanding on Tf-conjugated nanocarriers as efficient drug delivery system.

Protic additives determine the pathway of CdSe nanocrystal growth

The formation of semiconductor nanocrystals by hot-injection synthesis follows complex reaction mechanisms that are not yet fully understood. In particular the occurrence of intermediate species indicated by sharp, stationary spectral lines poses an important deviation from the predictions of classical nucleation theory. We show that trace amounts of water and other protic additives strongly impact the structure of these reaction intermediates, forming either coordination polymers under dry conditions or small clusters in the presence of moisture. These intermediates bind monomer during the initial nucleation phase. The structure of the intermediate determines the monomer release rate, either continuously or in a rapid dissolution event, and hence controls the reaction kinetics. From this we propose a kinetic model that allows us to predict secondary nucleation events. By directing the type of intermediate formed, protic additives provide a lever to manipulate this equilibrium and control nanocrystal synthesis in a rational fashion.

Quantum dots modified with quaternized poly(dimethylaminoethyl methacrylate) for selective recognition and killing of bacteria over mammalian cells

Copper-free click chemistry has been used to graft quaternized poly(dimethylaminoethyl methacrylate) (QPA) modified with azide to the quantum dots (QDs) derived with dibenzocyclooctynes (DBCO). The success of the quaternary ammonium polymer-modified QDs was confirmed by ultraviolet-visible spectrophotometry (UV-Vis), fluorescence spectroscopy, zeta (ζ) potential, size distribution, and transmission electron microscopy (TEM). The QPA-modified QDs exhibited properties of selective recognition and killing of bacteria. The novelty of this study lies in fact that the synthesis method of the antimicrobial QPA-modified QDs is simple. Moreover, from another standpoint, QPA-modified QDs simultaneously possess abilities of selective recognition and killing of bacteria over mammalian cells, which is very different from the currently designed multifunctional antimicrobial systems composed of complicated systematic compositions.

Clickable-Zwitterionic Copolymer Capped-Quantum Dots for in Vivo Fluorescence Tumor Imaging

In the last decades, fluorescent quantum dots (QDs) have appeared as high-performance biological fluorescent nanoprobes and have been explored for a variety of biomedical optical imaging applications. However, many central challenges still exist concerning the control of the surface chemistry to ensure high biocompatibility, low toxicity, antifouling, and specific active targeting properties. Regarding in vivo applications, circulation time and clearance of the nanoprobe are also key parameters to control the design and characterization of new optical imaging agents. Herein, the complete design and characterization of a peptide-near-infrared-QD-based nanoprobe for biomedical optical imaging is presented from the synthesis of the QDs and the zwitterionic-azide copolymer ligand, enabling a bio-orthogonal coupling, till the final in vivo test through all the characterization steps. The developed nanoprobes show high fluorescence emission, controlled grafting rate, low toxicity, in vitro active specific targeting, and in vivo long circulating blood time. This is, to our knowledge, the first report characterizing the in vivo circulation kinetics and tumor accumulation of targeted zwitterionic QDs.

Click-Functionalized SERS Nanoprobes with Improved Labeling Efficiency and Capability for Cancer Cell Imaging

Precise identification and detection of cancer cells using nanoparticle probes are critically important for early cancer diagnosis and subsequent therapy. We herein develop novel folate receptor (FR)-targeted surface-enhanced Raman scattering (SERS) nanoprobes for cancer cell imaging based on a click coupling strategy. A Raman-active derivative (5,5'-dithiobis(2-nitrobenzoic acid)-N₃ (DNBA-N₃)) is designed with a disulfide bond for covalently anchoring to the surface of hollow gold nanoparticles (HAuNPs) and a terminal azide group for facilitating highly efficient conjugation with the bioligand. Modification of HAuNPs with DNBA-N₃ yields monolayer coverage of Raman labels absorbed on the nanoparticle surface (HAuNP-DNBA-N₃) and strong SERS signals. HAuNP-DNBA-N₃ can be simply and effectively conjugated with folate bicyclo[6.1.0]nonyne derivatives via a copper-free click reaction. The synthesized nanoprobes (HAuNP-DNBA-folic acid (FA)) exhibit excellent targeted capacities to FR-positive cancer cells relative to FR-negative cells through SERS mappings. The receptor-mediated delivery behaviors are confirmed by comparison with the uptake of HAuNP-DNBA-N₃ and free FA competition experiments. In addition to its good stability and benign biocompatibility, the developed SERS nanoprobes have great potential for applications in targeted tumor imaging.

A novel type of quantum dot-transferrin conjugate using DNA hybridization mimics intracellular recycling of endogenous transferrin

Colloidal nanoparticles such as Quantum Dots (QDs) are promising alternatives to organic fluorophores, especially for long duration bioimaging. For specific targeting applications, QDs frequently require functionalization with selected proteins. In this regard, conjugation of proteins to QDs such that the nanobioconjugates retain the endogenous behavior of the coupled protein remains challenging. We have developed a novel method to conjugate a protein, transferrin (Tf), to QDs using DNA hybridization. These conjugates are characterized biochemically, and the trafficking properties in live cells are investigated. Although the internalization kinetics into the cells is much reduced compared to Tf labelled with organic dye, we could show that DNA hybridization-based QD-Tf conjugates are the first for which recycling from endosomes to the plasma membrane can be observed. This recycling occurs with kinetics that is similar to dye labelled Tf. We could image and follow the trajectories of recycling of individual vesicles for several tens of minutes. The conjugation of QDs to proteins mediated by DNA hybridization yields a new generation of ultra-bright and photostable probes that preserves the intracellular properties of the dye labelled protein better than previously reported QD conjugates using other surface chemistries for direct coupling.

Targeted glioma chemotherapy by cyclic RGD peptide-functionalized reversibly core-crosslinked multifunctional poly(ethylene glycol)-b-poly(ε-caprolactone) micelles

Cyclic RGD peptide-functionalized reversibly core-crosslinked biodegradable poly(ethylene glycol)-b-poly(ε-caprolactone) (PEG-PCL) micelles (cRGD-RCCMs) were designed and developed for highly potent and targeted glioma chemotherapy. To achieve crosslinkable core, dithiolane-functionalized trimethylene carbonate (DTC) was incorporated into PCL block. Interestingly, cRGD-RCCMs displayed a high doxorubicin (DOX) loading content of ∼18wt%, small hydrodynamic size of ∼50nm, and excellent colloidal stability with minimum drug leakage under physiological conditions while fast DOX release under cytoplasmic-mimicking reductive environments. MTT, confocal microscopy and flow cytometry measurement results pointed out that cRGD-RCCMs with 30% cRGD surface density (cRGD30-RCCMs) showed an evident selectivity, efficient cytoplasmic drug release, and superior antitumor activity to clinically used pegylated liposomal doxorubicin (DOX-LPs) in α_vβ₃ integrin overexpressing U87MG glioblastoma cells. Strikingly, DOX-loaded cRGD30-RCCMs demonstrated a prolonged circulation time showing an elimination half-life of ∼4.7h, three times exceeding that of the non-crosslinked counterparts, and a remarkably enhanced tumor accumulation of 7.7%ID/g. Furthermore, in vivo therapeutic studies revealed that DOX-loaded cRGD30-RCCMs effectively suppressed tumor growth, significantly prolonged survival time, and lessened side effects in subcutaneous U87MG glioblastoma-bearing nude mice. These reversibly core-crosslinked multifunctional biodegradable micelles might be developed into advanced and clinically viable targeted anticancer nanomedicines.

STATEMENT OF SIGNIFICANCE

Nanomedicines based on biodegradable micelles and nanoparticles offer a most promising treatment for malignant tumors. The therapeutic outcomes of current nanomedicines are, however, trimmed by their instability, low tumor retention, inefficient tumor cell uptake, and inferior drug release control. We report herein that cRGD-functionalized, rapidly glutathione-responsive, and reversibly core-crosslinked biodegradable micellar doxorubicin based on PEG-PCL block copolymer mediates potent and targeted glioma chemotherapy, affording significantly better treatment efficacy and lower systemic toxicity than the non-crosslinked micellar doxorubicin and clinically used pegylated liposomal doxorubicin controls. These reversibly core-crosslinked multifunctional biodegradable micelles have emerged as a robust, simple, versatile, and safe nanoplatform that might elegantly bridge the gap between the scientific and translational anticancer nanomedicine research.

Kinetic Analysis and Mechanism of the Hydrolytic Degradation of Squaramides and Squaramic Acids

The hydrolytic degradation of squaramides and squaramic acids, the product of partial hydrolysis of squaramides, has been evaluated by UV spectroscopy at 37 °C in the pH range 3-10. Under these conditions, the compounds are kinetically stable over long time periods (>100 days). At pH >10, the hydrolysis of the squaramate anions shows first-order dependence on both squaramate and OH^-. At the same temperature and [OH^-], the hydrolysis of squaramides usually displays biphasic spectral changes (A → B → C kinetic model) with formation of squaramates as detectable reaction intermediates. The measured rates for the first step (k₁ ≈ 10^-4 M^-1 s^-1) are 2-3 orders of magnitude faster than those for the second step (k₂ ≈ 10^-6 M^-1 s^-1). Experiments at different temperatures provide activation parameters with values of ΔH^⧧ ≈ 9-18 kcal mol^-1 and ΔS^⧧ ≈ -5 to -30 cal K^-1 mol^-1. DFT calculations show that the mechanism for the alkaline hydrolysis of squaramic acids is quite similar to that of amides.

Multifunctional and High Affinity Polymer Ligand that Provides Bio-Orthogonal Coating of Quantum Dots

We detail the design of hydrophilic metal-coordinating ligands and their use for the effective coating of luminescent quantum dots (QDs). The ligand design exploits the specific, reagent-free nucleophilic addition reaction of amine-modified molecules toward maleic anhydride to introduce several lipoic acid metal anchors, hydrophilic zwitterion moieties, and specific reactive groups along a poly(isobutylene-alt-maleic anhydride) (PIMA) chain. Tunable reactive groups tested in this study include azide, biotin, carboxyl, and amine. Cap exchange with these multilipoic acid ligands via a photochemical ligation strategy yields homogeneous QD dispersions that are colloidally stable over several biologically relevant conditions and for extended periods of time. The zwitterionic coating yields compact nanoparticle size and imparts nonsticky surface properties onto the QDs, preventing protein absorption. The introduction of a controllable number of reactive groups allows conjugation of the QDs to biomolecules via bio-orthogonal coupling chemistries including (1) attachment of the neurotransmitter dopamine to QDs via amine-isothiocyanate reaction to produce a platform capable of probing interactions with cysteine in proteins, based on charge transfer interactions; (2) self-assembly of biotinylated QDs with streptavidin-dye; and (3) ligation of azide-functionalized QDs to cyclooctyne-modified transferrin via copper-free click chemistry, used for intracellular delivery. This ligand design strategy can be used to prepare an array of metal-coordinating ligands adapted for coating other inorganic nanoparticles, including magnetic and plasmonic nanomaterials.

Quantum Dot Surface Engineering: Toward Inert Fluorophores with Compact Size and Bright, Stable Emission

The surfaces of colloidal nanocrystals are complex interfaces between solid crystals, coordinating ligands, and liquid solutions. For fluorescent quantum dots, the properties of the surface vastly influence the efficiency of light emission, stability, and physical interactions, and thus determine their sensitivity and specificity when they are used to detect and image biological molecules. But after more than 30 years of study, the surfaces of quantum dots remain poorly understood and continue to be an important subject of both experimental and theoretical research. In this article, we review the physics and chemistry of quantum dot surfaces and describe approaches to engineer optimal fluorescent probes for applications in biomolecular imaging and sensing. We describe the structure and electronic properties of crystalline facets, the chemistry of ligand coordination, and the impact of ligands on optical properties. We further describe recent advances in compact coatings that have significantly improved their properties by providing small hydrodynamic size, high stability and fluorescence efficiency, and minimal nonspecific interactions with cells and biological molecules. While major progress has been made in both basic and applied research, many questions remain in the chemistry and physics of quantum dot surfaces that have hindered key breakthroughs to fully optimize their properties.

Click Chemistry-Mediated Nanosensors for Biochemical Assays

Click chemistry combined with functional nanoparticles have drawn increasing attention in biochemical assays because they are promising in developing biosensors with effective signal transformation/amplification and straightforward signal readout for clinical diagnostic assays. In this review, we focus on the latest advances of biochemical assays based on Cu (I)-catalyzed 1, 3-dipolar cycloaddition of azides and alkynes (CuAAC)-mediated nanosensors, as well as the functionalization of nanoprobes based on click chemistry. Nanoprobes including gold nanoparticles, quantum dots, magnetic nanoparticles and carbon nanomaterials are covered. We discuss the advantages of click chemistry-mediated nanosensors for biochemical assays, and give perspectives on the development of click chemistry-mediated approaches for clinical diagnosis and other biomedical applications.

Investigations of ion transport through nanoscale polymer membranes by fluorescence quenching of CdSe/CdS quantum dot/quantum rods

Detailed steady-state and time-resolved fluorescence quenching measurements give deep insight into ion transport through nanometer thick diblock copolymer membranes, which were assembled as biocompatible shell material around CdSe/CdS quantum dot in quantum rods. We discuss the role of polymer chain length, intermolecular cross-linking and nanopore formation by analysing electron transfer processes from the photoexcited QDQRs to Cu(II) ions, which accumulate in the polymer membrane. Fluorescence investigations on single particle level additionally allow identifying ensemble inhomogeneities.

Surface labeling of enveloped virus with polymeric imidazole ligand-capped quantum dots via the metabolic incorporation of phospholipids into host cells

We report a general method for the preparation of quantum dot-labeled viruses through a strain-promoted azide-alkyne cycloaddition (SPAAC) reaction. The quantum dot sample was functionalized with methacrylate-based polymeric imidazole ligands (MA-PILs) bearing dibenzocyclooctyne groups. Enveloped measles virus was labeled with azide groups through the metabolic incorporation of a choline analogue into the host cell membrane, and then linked with the modified QDs. The virus retained its infectious ability against host cells after the modification with MA-PIL capped QDs.

Bio-orthogonal Coupling as a Means of Quantifying the Ligand Density on Hydrophilic Quantum Dots

We describe the synthesis of two metal-coordinating ligands that present one or two lipoic acid (LA) anchors, a hydrophilic polyethylene glycol (PEG) segment and a terminal reactive group made of an azide or an aldehyde, two functionalities with great utility in bio-orthogonal coupling techniques. These ligands were introduced onto the QD surfaces using a combination of photochemical ligation and mixed cap exchange strategy, where control over the fraction of azide and aldehyde groups per nanocrystal can be easily achieved: LA-PEG-CHO, LA-PEG-N3, and bis(LA)-PEG-CHO. We then demonstrate the application of two novel bio-orthogonal coupling strategies directly on luminescent quantum dot (QD) surfaces that use click chemistry and hydrazone ligation under catalyst-free conditions. We applied the highly efficient hydrazone ligation to couple 2-hydrozinopyridine (2-HP) to aldehyde-functionalized QDs, which produces a stable hydrazone chromophore with a well-defined optical signature. This unique optical feature has enabled us to extract a measure for the ligand density on the QDs for a few distinct sizes and for different ligand architectures, namely mono-LA-PEG and bis(LA)-PEG. We found that the foot-print-area per ligand was unaffected by the nanocrystal size but strongly depended on the ligand coordination number. Additionally, we showed that when the two bio-orthogonal functionalities (aldehyde and azide) are combined on the same QD platform, the nanocrystal can be specifically reacted with two distinct targets and with great specificity. This design yields QD platforms with distinct chemoselectivities that are greatly promising for use as carriers for in vivo imaging and delivery.

Synthesis of luminescent squaramide monoesters: cytotoxicity and cell imaging studies in HeLa cells

Luminescent squaramide monoesters functionalised with fluorophore groups have been explored as cytotoxic and imaging agents. The biodistribution behaviour differs depending on the fluorescent moiety; lysosomal and nuclear localisation have been observed.

Orthogonal dual-click diyne for CuAAC and/or SPAAC couplings

A diyne reagent capable of participating in CuAAC and/or SPAAC couplings in either sequential order is reported.

Squaramides with cytotoxic activity against human gastric carcinoma cells HGC-27: synthesis and mechanism of action

A series of squaramates and squaramides have been synthesized and their cytotoxic activity has been investigated in different cancer cell lines.

An engineered coiled-coil polypeptide assembled onto quantum dots for targeted cell imaging

Quantum dot (QD)-polypeptide probes have been developed through the specific metal-affinity interaction between polypeptides appended with N-terminal polyhistidine sequences and CdSe/ZnS core-shell QDs. The size and charge of a QD-polypeptide can be tuned by using different coiled-coil polypeptides. Compared to glutathione-capped QDs (QD-GSH), QD-polypeptide probes showed an approximately two- to three-fold luminescence increase, and the luminescence increase was not obviously related to the charge of the polypeptide. QD-polypeptide probes with different charge have a great effect on nonspecific cellular uptake. QD-polypeptide probes with negative charge exhibited lower nonspecific cellular uptake in comparison to the QD-GSH, while positively charged QD-polypeptide probes presented higher cellular uptake than the QD-GSH. A targeted QD-ARGD probe can obviously increase targeted cellular uptake in α v β 3 overexpressing HeLa cells compared to QD-A. In addition, QD-polypeptide probes showed lower in vitro cytotoxicity compared to the original QDs. These results demonstrate that these QD-polypeptide probes with high specific cellular uptake, high fluorescence intensity and low background noise are expected to have great potential applications in targeted cell imaging.

Protein Labelling with Versatile Phosphorescent Metal Complexes for Live Cell Luminescence Imaging

To take advantage of the luminescent properties of d(6) transition metal complexes to label proteins, versatile bifunctional ligands were prepared. Ligands that contain a 1,2,3-triazole heterocycle were synthesised using Cu(I) catalysed azide-alkyne cycloaddition "click" chemistry and were used to form phosphorescent Ir(III) and Ru(II) complexes. Their emission properties were readily tuned, by changing either the metal ion or the co-ligands. The complexes were tethered to the metalloprotein transferrin using several conjugation strategies. The Ir(III)/Ru(II)-protein conjugates could be visualised in cancer cells using live cell imaging for extended periods without significant photobleaching. These versatile phosphorescent protein-labelling agents could be widely applied to other proteins and biomolecules and are useful alternatives to conventional organic fluorophores for several applications.

Label-free screening of single biomolecules through resistive pulse sensing technology for precision medicine applications

Employing integrated nano- and microfluidic circuits for detecting and characterizing biological compounds through resistive pulse sensing technology is a vibrant area of research at the interface of biotechnology and nanotechnology. Resistive pulse sensing platforms can be customized to study virtually any particle of choice which can be threaded through a fluidic channel and enable label-free single-particle interrogation with the primary read-out signal being an electric current fingerprint. The ability to perform label-free molecular screening with single-molecule and even single binding site resolution makes resistive pulse sensing technology a powerful tool for analyzing the smallest units of biological systems and how they interact with each other on a molecular level. This task is at the core of experimental systems biology and in particular 'omics research which in combination with next-generation DNA-sequencing and next-generation drug discovery and design forms the foundation of a novel disruptive medical paradigm commonly referred to as personalized medicine or precision medicine. DNA-sequencing has approached the 1000-Dollar-Genome milestone allowing for decoding a complete human genome with unmatched speed and at low cost. Increased sequencing efficiency yields massive amounts of genomic data. Analyzing this data in combination with medical and biometric health data eventually enables understanding the pathways from individual genes to physiological functions. Access to this information triggers fundamental questions for doctors and patients alike: what are the chances of an outbreak for a specific disease? Can individual risks be managed and if so how? Which drugs are available and how should they be applied? Could a new drug be tailored to an individual's genetic predisposition fast and in an affordable way? In order to provide answers and real-life value to patients, the rapid evolvement of novel computing approaches for analyzing big data in systems genomics has to be accompanied by an equally strong effort to develop next-generation DNA-sequencing and next-generation drug screening and design platforms. In that context lab-on-a-chip devices utilizing nanopore- and nanochannel based resistive pulse-sensing technology for DNA-sequencing and protein screening applications occupy a key role. This paper describes the status quo of resistive pulse sensing technology for these two application areas with a special focus on current technology trends and challenges ahead.

A step towards mobile arsenic measurement for surface waters

Surface modified quantum dots (QDs) are studied using a bio-inspired cysteine rich ligand (glutathione, GSH) and their quenching response and selectivity to arsenic examined. As predicted from As(3+) binding with highly crosslinked phytochelatin-(PCn)-like molecules, better arsenic selectivity is obtained for a thicker more 3-dimensional GSH surface layer, with exposed sulfhydryl groups. A detection limit of at least 10 μM can be achieved using CdSe/ZnS core-shell QDs capped with this GSH structure. The system is also demonstrated using a mobile phone camera to record the measurement, producing a detection limit of 5 μM. However, copper remains the main interferent of concern. Water-soluble CdTe QDs show little sensitivity to As(3+) even with a GSH surface, but they remain sensitive to Cu(2+), allowing a copper baseline to be established from the CdTe measurement. Despite anticipating that spectrally non overlapping fluorescence would be required from the two types of QDs to achieve this, a method is demonstrated using RGB channels from a mobile phone and processing the raw data for CdTe QDs, with an emission wavelength of 600 nm, and CdSe/ZnS QDs, with emission maximum of 630 nm. It is shown that As(3+) measurement remains feasible at the WHO guideline value of 10 μg L(-1) up to a copper concentration of around 0.3 μM Cu(2+), which corresponds to the highest recorded level in a selection of large rivers world-wide.

Quantum dots for quantitative imaging: from single molecules to tissue

Since their introduction to biological imaging, quantum dots (QDs) have progressed from a little known, but attractive, technology to one that has gained broad application in many areas of biology. The versatile properties of these fluorescent nanoparticles have allowed investigators to conduct biological studies with extended spatiotemporal capabilities that were previously not possible. In this review, we focus on QD applications that provide enhanced quantitative information concerning protein dynamics and localization, including single particle tracking and immunohistochemistry, and finish by examining the prospects of upcoming applications, such as correlative light and electron microscopy and super-resolution. Advances in single molecule imaging, including multi-color and three-dimensional QD tracking, have provided new insights into the mechanisms of cell signaling and protein trafficking. New forms of QD tracking in vivo have allowed the observation of biological processes at molecular level resolution in the physiological context of the whole animal. Further methodological development of multiplexed QD-based immunohistochemistry assays should enable more quantitative analysis of key proteins in tissue samples. These advances highlight the unique quantitative data sets that QDs can provide to further our understanding of biological and disease processes.

Quantum dots: bright and versatile in vitro and in vivo fluorescence imaging biosensors

Colourful cells and tissues: semiconductor quantum dots and their versatile applications in multiplexed bioimaging research.

Light transfer from quantum-dot-doped polymer nanowires to silver nanowires

The plasmons of two silver nanowires are simultaneously excited by photoluminescence of the quantum-dot-doped nanowire under 532 nm laser excitation.

Strategies for interfacing inorganic nanocrystals with biological systems based on polymer-coating

A representative set of nanocrystals made of semiconductors, Au and iron oxide, surface-capped with polymer ligands presenting various metal-coordinating groups.

Multifunctional quantum dot-polypeptide hybrid nanogel for targeted imaging and drug delivery

A new type of multifunctional quantum dot (QD)-polypeptide hybrid nanogel with targeted imaging and drug delivery properties has been developed by metal-affinity driven self-assembly between artificial polypeptides and CdSe-ZnS core-shell QDs. On the surface of QDs, a tunable sandwich-like microstructure consisting of two hydrophobic layers and one hydrophilic layer between them was verified by capillary electrophoresis, transmission electron microscopy, and dynamic light scattering measurements. Hydrophobic and hydrophilic drugs can be simultaneously loaded in a QD-polypeptide nanogel. In vitro drug release of drug-loaded QD-polypeptide nanogels varies strongly with temperature, pH, and competitors. A drug-loaded QD-polypeptide nanogel with an arginine-glycine-aspartic acid (RGD) motif exhibited efficient receptor-mediated endocytosis in αvβ3 overexpressing HeLa cells but not in the control MCF-7 cells as analyzed by confocal microscopy and flow cytometry. In contrast, non-targeted QD-polypeptide nanogels revealed minimal binding and uptake in HeLa cells. Compared with the original QDs, the QD-polypeptide nanogels showed lower in vitro cytotoxicity for both HeLa cells and NIH 3T3 cells. Furthermore, the cytotoxicity of the targeted QD-polypeptide nanogel was lower for normal NIH 3T3 cells than that for HeLa cancer cells. These results demonstrate that the integration of imaging and drug delivery functions in a single QD-polypeptide nanogel has the potential for application in cancer diagnosis, imaging, and therapy.