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• W3132295265 abstract "Fluorescence imaging has gathered interest over the recent years for its real-time response and high sensitivity. Developing probes for this modality has proven to be a challenge. Quantum dots (QDs) are colloidal nanoparticles that possess unique optical and electronic properties due to quantum confinement effects, whose excellent optical properties make them ideal for fluorescence imaging of biological systems. By selectively controlling the synthetic methodologies it is possible to obtain QDs that emit in the first (650–950 nm) and second (1000–1400 nm) near infra-red (NIR) windows, allowing for superior imaging properties. Despite the excellent optical properties and biocompatibility shown by some NIR QDs, there are still some challenges to overcome to enable there use in clinical applications. In this review, we discuss the latest advances in the application of NIR QDs in preclinical settings, together with the synthetic approaches and material developments that make NIR QDs promising for future biomedical applications. Fluorescence imaging has gathered interest over the recent years for its real-time response and high sensitivity. Developing probes for this modality has proven to be a challenge. Quantum dots (QDs) are colloidal nanoparticles that possess unique optical and electronic properties due to quantum confinement effects, whose excellent optical properties make them ideal for fluorescence imaging of biological systems. By selectively controlling the synthetic methodologies it is possible to obtain QDs that emit in the first (650–950 nm) and second (1000–1400 nm) near infra-red (NIR) windows, allowing for superior imaging properties. Despite the excellent optical properties and biocompatibility shown by some NIR QDs, there are still some challenges to overcome to enable there use in clinical applications. In this review, we discuss the latest advances in the application of NIR QDs in preclinical settings, together with the synthetic approaches and material developments that make NIR QDs promising for future biomedical applications. Current imaging techniques used in a clinical setting include magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET), single-photon emission computed tomography (SPECT), and ultrasound. Each of these techniques has strengths and limitations; for example, MRI can produce high-resolution anatomical and functional images, with high tissue penetration, without using ionizing radiation, yet suffers from poor image contrast. PET offers a highly sensitive imaging modality, yet suffers from poor resolution, requires ionizing radiation, and will usually be paired with an anatomical technique such as MRI or CT (Judenhofer et al., 2008Judenhofer M.S. Wehrl H.F. Newport D.F. Catana C. Siegel S.B. Becker M. Thielscher A. Kneilling M. Lichy M.P. Eichner M. Simultaneous pet-mri: a new approach for functional and morphological imaging.Nat. Med. 2008; 14: 459-465Crossref PubMed Scopus (734) Google Scholar). Furthermore, most of these imaging techniques require large, expensive machines to produce these images, have scanning times on the order of minutes to hours, and have significant image reconstruction periods associated with them (Alkhybari et al., 2018Alkhybari E.M. McEntee M.F. Brennan P.C. Willowson K.P. Hogg P. Kench P.L. Determining and updating pet/ct and spect/ct diagnostic reference levels: a systematic review.Radiat. Prot. Dosimetry. 2018; 182: 532-545Crossref PubMed Scopus (7) Google Scholar; Leigh et al., 2002Leigh P.N. Simmons A. Williams S. Williams V. Turner M. Brooks D. Imaging: Mrs/mri/pet/spect: Summary.Amyotroph. Lateral Scler. Other Mot. Neuron Disord. 2002; 3: S75-S80Crossref PubMed Google Scholar). Ultrasound stands out in contrast to this, allowing for real-time image acquisition and reconstruction using handheld equipment; however, it has limitations in terms of tissue penetration and image contrast. Fluorescence imaging is an alternative modality that can offer real-time imaging with high contrast and resolution, although it has similar issues with tissue penetration (Cao et al., 2019Cao H. Yue Z. Gao H. Chen C. Cui K. Zhang K. Cheng Y. Shao G. Kong D. Li Z. In vivo real-time imaging of extracellular vesicles in liver regeneration via aggregation-induced emission luminogens.ACS Nano. 2019; 13: 3522-3533Crossref PubMed Scopus (22) Google Scholar; Huang et al., 2019Huang J. Xie C. Zhang X. Jiang Y. Li J. Fan Q. Pu K. Renal-clearable molecular semiconductor for second near-infrared fluorescence imaging of kidney dysfunction.Angew. Chem. Int. Ed. 2019; 58: 15120-15127Crossref PubMed Scopus (70) Google Scholar; Zhang et al., 2019aZhang A. Pan S. Zhang Y. Chang J. Cheng J. Huang Z. Li T. Zhang C. de la Fuentea J.M. Zhang Q. Carbon-gold hybrid nanoprobes for real-time imaging, photothermal/photodynamic and nanozyme oxidative therapy.Theranostics. 2019; 9: 3443Crossref PubMed Scopus (29) Google Scholar). Fluorescent imaging typically involves the administration of an exogenous probe to allow for signal generation. Excitation of these agents through the application of light of an appropriate wavelength causes them to fluoresce; collection of this emitted signal produces the image. This imaging modality is of great benefit for diagnosis, with preclinical applications for tumor identification and brain injury being demonstrated (Li et al., 2020Li C. Li W. Liu H. Zhang Y. Chen G. Li Z. Wang Q. An activatable nir-ii nanoprobe for in vivo early real-time diagnosis of traumatic brain injury.Angew. Chem. 2020; 132: 253-258Crossref Google Scholar). It has also been shown to be applicable to single-cell tracking (Awasthi et al., 2020Awasthi P. An X. Xiang J. Kalva N. Shen Y. Li C. Facile synthesis of noncytotoxic pegylated dendrimer encapsulated silver sulfide quantum dots for nir-ii biological imaging.Nanoscale. 2020; 12: 5678-5684Crossref PubMed Google Scholar) for studying the immune system and circulating tumor cells and has also been applied to fluorescence-guided surgery (Tian et al., 2020Tian R. Ma H. Zhu S. Lau J. Ma R. Liu Y. Lin L. Chandra S. Wang S. Zhu X. Multiplexed NIR-II probes for lymph node-invaded cancer detection and imaging-guided surgery.Adv. Mater. 2020; 32: 1907365Crossref Scopus (13) Google Scholar). When focusing on the biological applications of fluorescent probes, and imaging with these probes, there are ideal characteristics that they should possess. They must be selective for the target, possess low toxicity for the organism, and have excellent fluorescent properties (i.e. high photoluminescent quantum yield [PLQY], photostability, tuneable emission) that allow visualization of the probe's localization/distribution with a reasonable tissue penetration depth. Recent advances in fluorescent imaging have pushed the maximum reported tissue depth, from 3.2 cm to 8 cm, and allowed for the whole imaging of a live rat and also the tracking of 100 μm particles through the gastrointestinal tract of a live mouse (Dang et al., 2019Dang X. Bardhan N.M. Qi J. Gu L. Eze N.A. Lin C.-W. Kataria S. Hammond P.T. Belcher A.M. Deep-tissue optical imaging of near cellular-sized features.Sci. Rep. 2019; 9: 1-12Crossref PubMed Scopus (10) Google Scholar). Fluorescent probes can be small organic molecules (e.g. cyanine, rhodamine, or BODIPY), metal complexes (e.g. Ir, Ru, Ln), fluorescent proteins (e.g. GFP), polymers, or nanoparticles. A key part of the fluorescent probe is the organic dye. Unfortunately, many organic dyes have the significant limitation that their excitation and emission lie within the ultraviolet (UV)-visible region, outside the biological transparency windows. In biological applications the exciting or emitted light may be absorbed by water, hemoglobin, oxygenated hemoglobin, skin, fat, proteins, and cell structures (Hemmer et al., 2016Hemmer E. Benayas A. Légaré F. Vetrone F. Exploiting the biological windows: current perspectives on fluorescent bioprobes emitting above 1000 nm.Nanoscale Horiz. 2016; 1: 168-184Crossref PubMed Google Scholar; Zhao et al., 2018Zhao P. Xu Q. Tao J. Jin Z. Pan Y. Yu C. Yu Z. Near infrared quantum dots in biomedical applications: current status and future perspective.Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 2018; 10: e1483Crossref PubMed Scopus (81) Google Scholar). This hampers the use of this technique and limits its application to regions close to the surface of the sample or patient being monitored when using light in the UV-visible range (Zhang et al., 2016Zhang H. Salo D.C. Kim D.M. Komarov S. Tai Y.-C. Berezin M.Y. Penetration depth of photons in biological tissues from hyperspectral imaging in shortwave infrared in transmission and reflection geometries.J. Biomed. Opt. 2016; 21: 126006Crossref PubMed Scopus (50) Google Scholar). Furthermore, there is a background signal, or autofluorescence, produced by tissues within the body when excitation wavelengths within the UV-visible range are used; this reduces the contrast and image quality obtained. The near-infrared (NIR) region of the electromagnetic spectrum is seen as ideal for fluorescence imaging. The scattering, absorption, and auto-fluorescence from tissues is greatly reduced in this region. This should allow for improved image quality and greater tissue penetration, broadening the applications of this technique for diagnosis of disease and monitoring of treatment. The NIR region can be further separated into NIR-I (between 650 and 950 nm) and NIR-II (between 1000 and 1400 nm), the so-called NIR biological windows. It is within these windows that the interference of biological media is lowest (Figure 1). Quantum dots (QDs) are a subset of nanoparticles and given their properties, can be used as fluorescent NIR probes, as an alternative to organic dyes. QDs are typically inorganic semiconductor nanoparticles, with the exception of carbon QDs, with sizes usually below 50 nm. The distinct optical properties observed in these compounds, when compared with their bulk counterparts, are due to the existence of quantum confinement effects. The quantum confinement effects that are observed are directly influenced by the material's composition and physical dimensions. In bulk semiconductor materials there is a band-gap energy (Eg) that represents the difference between the highest occupied energy state of the valence band and the lowest unoccupied state of the conduction band. When an electron in the valence band absorbs a photon with energy equal to, or higher than Eg, it becomes excited and transitions to the conduction band. This leaves behind an “electron hole.” The negatively charged electron and the positively charged hole form a quasi-particle called an exciton. When the electron relaxes back to the valence band, it annihilates the exciton and may release energy in the form of a photon, with energy lower than Eg (due to energy lost in the transition), in a process called radiative recombination. The exciton has a finite size within a crystal that is defined as the exciton Bohr radius. When the size of the semiconductor crystal is smaller than the size of the exciton Bohr radius, quantum confinement effects arise, the charge carriers become spatially confined, and the energy levels become discrete. Therefore, the exciton Bohr radius defines the transition point between the different properties observed in the bulk state and in the quantum confined state. In the latter, small variations in the size of the nanocrystals lead to size-dependent absorption and emission profiles. The control of the particle size becomes the handle by which the optical properties of the QD can be tuned (Buhro and Colvin, 2003Buhro W.E. Colvin V.L. Shape matters.Nat. Mater. 2003; 2: 138-139Crossref PubMed Google Scholar). As a consequence of these interesting characteristics, QDs have been extensively studied and have found applications in different areas such as optoelectronic devices, biological imaging, and solar energy devices (Jamieson et al., 2007Jamieson T. Bakhshi R. Petrova D. Pocock R. Imani M. Seifalian A.M. Biological applications of quantum dots.Biomaterials. 2007; 28: 4717-4732Crossref PubMed Scopus (0) Google Scholar). QDs probes have some unique properties that make them particularly favorable for fluorescent imaging applications. These include•Size-modulated absorbance and emission, allowing for the production of probes made of the same material with different optical properties for multiplexing purposes (Yu et al., 2019Yu G.-T. Luo M.-Y. Li H. Chen S. Huang B. Sun Z.-J. Cui R. Zhang M. Molecular targeting nanoprobes with non-overlap emission in the second near-infrared window for in vivo two-color colocalization of immune cells.ACS Nano. 2019; 13: 12830-12839Crossref PubMed Scopus (5) Google Scholar; Zhao et al., 2018Zhao P. Xu Q. Tao J. Jin Z. Pan Y. Yu C. Yu Z. Near infrared quantum dots in biomedical applications: current status and future perspective.Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 2018; 10: e1483Crossref PubMed Scopus (81) Google Scholar).•High photostability, allowing imaging for extended periods of time without loss of signal—potentially favorable for fluorescence-guided surgery (Jain et al., 2008Jain P.K. Huang X. El-Sayed I.H. El-Sayed M.A. Noble metals on the nanoscale: optical and photothermal properties and some applications in imaging, sensing, biology, and medicine.Acc. Chem. Res. 2008; 41: 1578-1586Crossref PubMed Scopus (3049) Google Scholar; Zhao et al., 2018Zhao P. Xu Q. Tao J. Jin Z. Pan Y. Yu C. Yu Z. Near infrared quantum dots in biomedical applications: current status and future perspective.Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 2018; 10: e1483Crossref PubMed Scopus (81) Google Scholar).•Large Stokes shift: this simplifies filtering out the excitation light and also allows for multicolor imaging using a single excitation wavelength (Yu et al., 2019Yu G.-T. Luo M.-Y. Li H. Chen S. Huang B. Sun Z.-J. Cui R. Zhang M. Molecular targeting nanoprobes with non-overlap emission in the second near-infrared window for in vivo two-color colocalization of immune cells.ACS Nano. 2019; 13: 12830-12839Crossref PubMed Scopus (5) Google Scholar; Zhao et al., 2018Zhao P. Xu Q. Tao J. Jin Z. Pan Y. Yu C. Yu Z. Near infrared quantum dots in biomedical applications: current status and future perspective.Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 2018; 10: e1483Crossref PubMed Scopus (81) Google Scholar).•High brightness resulting from their large excitation cross-section, allowing for improved sensitivity, which in turn allows for imaging at the receptor concentration level (Zhao et al., 2018Zhao P. Xu Q. Tao J. Jin Z. Pan Y. Yu C. Yu Z. Near infrared quantum dots in biomedical applications: current status and future perspective.Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 2018; 10: e1483Crossref PubMed Scopus (81) Google Scholar).•Despite having similar PLQYs in the UV-visible range, QDs have superior PLQYs to organic dyes in the NIR range (Resch-Genger et al., 2008Resch-Genger U. Grabolle M. Cavaliere-Jaricot S. Nitschke R. Nann T. Quantum dots versus organic dyes as fluorescent labels.Nat. Methods. 2008; 5: 763Crossref PubMed Scopus (2704) Google Scholar).•Longer lifetime of the excited state, potentially allowing for the development of probes that can be used for time-delayed microscopy to avoid the autofluorescence of cells and other moieties (Pons et al., 2019Pons T. Bouccara S. Loriette V. Lequeux N. Pezet S. Fragola A. In vivo imaging of single tumor cells in fast-flowing bloodstream using near-infrared quantum dots and time-gated imaging.ACS Nano. 2019; 13: 3125-3131Crossref PubMed Scopus (16) Google Scholar; Resch-Genger et al., 2008Resch-Genger U. Grabolle M. Cavaliere-Jaricot S. Nitschke R. Nann T. Quantum dots versus organic dyes as fluorescent labels.Nat. Methods. 2008; 5: 763Crossref PubMed Scopus (2704) Google Scholar).•High-surface area-to-volume ratio that allows for efficient functionalization with other imaging agents, creating a multimodal probe, allowing imaging of disease states at different scale lengths and different tissue depths (Stasiuk et al., 2011Stasiuk G.J. Tamang S. Imbert D. Poillot C. Giardiello M. Tisseyre C. Barbier E.L. Fries P.H. De Waard M. Reiss P. Cell-permeable ln(III) chelate-functionalized inp quantum dots as multimodal imaging agents.ACS Nano. 2011; 5: 8193-8201Crossref PubMed Scopus (0) Google Scholar). NIR QDs are a specific class of fluorescent probes that emit in the NIR region of the spectra and stand as one of the most promising materials used in pre-clinical setting. Their small size (up to 30 nm), excellent PLQY (up to 45% in aqueous media), high photostability, and biocompatibility are their main advantages over the traditional molecular dyes (Resch-Genger et al., 2008Resch-Genger U. Grabolle M. Cavaliere-Jaricot S. Nitschke R. Nann T. Quantum dots versus organic dyes as fluorescent labels.Nat. Methods. 2008; 5: 763Crossref PubMed Scopus (2704) Google Scholar). Furthermore, the possibility to load the surface of these QDs with specific targeting motifs and therapeutic agents creates the possibility to engineer a whole new class of theranostics with enhanced properties (Li et al., 2019cLi Y. Bai G. Zeng S. Hao J. Theranostic carbon dots with innovative nir-ii emission for in vivo renal-excreted optical imaging and photothermal therapy.ACS Appl. Mater. Interface. 2019; 11: 4737-4744Crossref PubMed Scopus (0) Google Scholar; Liu et al., 2020aLiu H. Li C. Qian Y. Hu L. Fang J. Tong W. Nie R. Chen Q. Wang H. Magnetic-induced graphene quantum dots for imaging-guided photothermal therapy in the second near-infrared window.Biomaterials. 2020; 232: 119700Crossref PubMed Scopus (23) Google Scholar; Zhao et al., 2020Zhao Y. Song M. Yang X. Yang J. Du C. Wang G. Yi J. Shan G. Li D. Liu L. Amorphous Ag2-xCuxS quantum dots:“All-in-one” theranostic nanomedicines for near-infrared fluorescence/photoacoustics dual-modal-imaging-guided photothermal therapy.Chem. Eng. J. 2020; : 125777Crossref Scopus (0) Google Scholar). Thus, further development of QDs as NIR probes for biomedical imaging could be the key to future success of this imaging modality. In this review we discuss the different methodologies through which QDs are synthesized and the different classes of materials that compose these QDs, with a focus on NIR QDs. After establishing the materials used in these nanoparticles, we discuss the state-of-the-art in biomedical fluorescence imaging, highlighting the possibility for image-guided surgery with NIR QDs. In a final remark, we highlight the future perspectives and challenges that the field still has to overcome to see the NIR QDs implemented in a clinical setting. Since the discovery of an efficient method to synthesize QDs in 1993, Cd(II) and Pb(II) have been used as the base metal for the fluorescent core of the nanoparticles (Murray et al., 1993Murray C. Norris D.J. Bawendi M.G. Synthesis and characterization of nearly monodisperse CdE (E= sulfur, selenium, tellurium) semiconductor nanocrystallites.J. Am. Chem. Soc. 1993; 115: 8706-8715Crossref Google Scholar). Although the QDs obtained show excellent properties and NIR emission, toxicity problems due to the leaching of heavy metals have been a major drawback for their application in biological systems (Allocca et al., 2019Allocca M. Mattera L. Bauduin A. Miedziak B. Moros M. De Trizio L. Tino A. Reiss P. Ambrosone A. Tortiglione C. An integrated multilevel analysis profiling biosafety and toxicity induced by indium-and cadmium-based quantum dots in vivo.Environ. Sci. Technol. 2019; 53: 3938-3947Crossref PubMed Scopus (0) Google Scholar; Brunetti et al., 2013Brunetti V. Chibli H. Fiammengo R. Galeone A. Malvindi M.A. Vecchio G. Cingolani R. Nadeau J.L. Pompa P.P. InP/ZnS as a safer alternative to CdSe/ZnS core/shell quantum dots: in vitro and in vivo toxicity assessment.Nanoscale. 2013; 5: 307-317Crossref PubMed Scopus (0) Google Scholar). In recent years, the synthesis of NIR QDs has evolved toward the integration of new materials such as Ag(I), Cu(I), and more recently carbon dots (Li and Wu, 2019aLi C. Wu P. Cu-doped quantum dots: a new class of near-infrared emitting fluorophores for bioanalysis and bioimaging.Luminescence. 2019; 34: 782-789Crossref PubMed Scopus (3) Google Scholar; Li et al., 2019cLi Y. Bai G. Zeng S. Hao J. Theranostic carbon dots with innovative nir-ii emission for in vivo renal-excreted optical imaging and photothermal therapy.ACS Appl. Mater. Interface. 2019; 11: 4737-4744Crossref PubMed Scopus (0) Google Scholar; Zhang et al., 2020Zhang Y. Yang H. An X. Wang Z. Yang X. Yu M. Zhang R. Sun Z. Wang Q. Controlled synthesis of [email protected] core–shell quantum dots with enhanced and tunable fluorescence in the second near-infrared window.Small. 2020; 16: 2001003Crossref Scopus (4) Google Scholar). These present safer alternatives to the classical QDs, while even improving upon some of the photophysical properties. The synthetic methodologies that have been developed up to the present day focus on surface modifications as means to improve the biological compatibility and at the same time increase the fluorescent properties by shifting the emission toward longer wavelengths. The main synthetic methods used in the synthesis of NIR QDs are hot-injection, heat-up, microwave, and hydrothermal synthesis (Figure 2). They possess different advantages and are used in different circumstances depending on the nature of reagents, solvents, and the desired coating of the final product. In-depth reviews on these synthetic methodologies have been conducted (Reiss et al., 2016Reiss P. Carriere M. Lincheneau C. Vaure L. Tamang S. Synthesis of semiconductor nanocrystals, focusing on nontoxic and earth-abundant materials.Chem. Rev. 2016; 116: 10731-10819Crossref PubMed Scopus (271) Google Scholar; Tamang et al., 2016Tamang S. Lincheneau C. Hermans Y. Jeong S. Reiss P. Chemistry of InP nanocrystal syntheses.Chem. Mater. 2016; 28: 2491-2506Crossref Scopus (167) Google Scholar; van Embden et al., 2015van Embden J. Chesman A.S. Jasieniak J.J. The heat-up synthesis of colloidal nanocrystals.Chem. Mater. 2015; 27: 2246-2285Crossref Scopus (203) Google Scholar), and in this section we present a summary, for a better understanding of the following sections. The hot-injection method (Figure 2A) was first introduced by the works of Murray and co-workers (Murray et al., 1993Murray C. Norris D.J. Bawendi M.G. Synthesis and characterization of nearly monodisperse CdE (E= sulfur, selenium, tellurium) semiconductor nanocrystallites.J. Am. Chem. Soc. 1993; 115: 8706-8715Crossref Google Scholar), and it remains the most common method used to synthesize a wide variety of monodisperse QDs. Despite the benefits and the control that a hot-injection synthesis offers for the production of QDs, this method still suffers from some drawbacks. The high temperature of the reaction results in rapid reaction rates; as such, mixing of the reagents must be efficient to produce monodisperse nanoparticles. This is more difficult to achieve as the reaction volume increases; when the volume crosses a threshold limit the homogeneity of the batch is less reproducible, making this method less suitable for large-scale QD production than other methods. The heat-up synthetic method (Figure 2B) for QDs is a single-pot reaction without an injection step. The nucleation period in this technique is usually spread over a long period of time, and because of this some precursors start to nucleate at different time points (van Embden et al., 2015van Embden J. Chesman A.S. Jasieniak J.J. The heat-up synthesis of colloidal nanocrystals.Chem. Mater. 2015; 27: 2246-2285Crossref Scopus (203) Google Scholar). This leads to an increased polydispersity in size distribution. Another major drawback of this technique is the need to employ reagents that have similar reactivities at the desired reaction temperatures (Chen et al., 2016Chen S. Ahmadiantehrani M. Zhao J. Zhu S. Mamalis A.G. Zhu X. Heat-up synthesis of Ag–In–S and Ag–In–S/ZnS nanocrystals: effect of indium precursors on their optical properties.J. Alloys Compd. 2016; 665: 137-143Crossref PubMed Scopus (9) Google Scholar). Matching the reactivity of the ligands and precursors ensures that the nucleation occurs in a short window and generates enough nuclei. The search for a larger library of reagents and solvents is thus one of the priorities for the advancement of this technique. The microwave method (Figure 2C) makes use of electromagnetic radiation to achieve a rapid and homogeneous heating of the reaction. The control over the rate of heating during the synthesis offers the potential for better control over the formation of the QDs (Sinatra et al., 2017Sinatra L. Pan J. Bakr O.M. Methods of synthesizing monodisperse colloidal quantum dots.Mater. Lett. 2017; 12: 3-7Google Scholar), which have been exploited to improve monodispersity in a reproducible manner. These intriguing properties make this method a viable alternative to hot-injection methodology with respect to the monodispersity of the QDs. Nonetheless, this method is still in its early stages of development and the QDs obtained usually have low PLQY, hindering their applications. The requirement for a polarizable solvent makes the direct comparison to other methods difficult, as the aliphatic solvents used are not suitable for microwave-based synthetic procedures (Sun et al., 2016Sun J. Wang W. Yue Q. Review on microwave-matter interaction fundamentals and efficient microwave-associated heating strategies.Materials. 2016; 9: 231Crossref Scopus (157) Google Scholar). The hydrothermal method (Figure 2D) refers to the use of aqueous solvents at high temperature and pressure for the synthesis of QDs (Sonawane et al., 2018Sonawane G.H. Patil S.P. Sonawane S.H. Nanocomposites and its applications.in: Applications of Nanomaterials. Elsevier, 2018: 1-22Crossref Google Scholar). The synergetic effect of these conditions leads to the rapid formation of nuclei, resulting in monodisperse QDs that do not require post-synthesis treatments in order to be compatible with aqueous media. The hydrothermal method is often preferred when the final application of the synthesized QDs is biological. It is the most employed variation of aqueous synthesis of QDs, although the drawback of this method is the weak photophysical properties of the QDs obtained. Post-treatment with methods such as size-selective precipitation, photochemical etching, and surface modifications are usually employed in order to improve their properties (Wei et al., 2018Wei X. Al Muyeed S.A. Peart M.R. Sun W. Tansu N. Wierer Jr., J.J. Room temperature luminescence of passivated ingan quantum dots formed by quantum-sized-controlled photoelectrochemical etching.Appl. Phys. Lett. 2018; 113: 121106Crossref Scopus (5) Google Scholar). All of the techniques above represent a different method of obtaining QDs, with advantages and disadvantages. Hot-injection and heat-up methods are the most used and consist of using high temperatures in order to control the nucleation and growth of QDs, but the reaction conditions are difficult to precisely control and reproducibility is hard to achieve. Hydrothermal and microwave are faster and enable a better control of the reaction conditions, but further optimizations are still required to achieve better optical properties of the synthesized QDs. The methodologies described in the previous section have been used to synthesize QDs with different materials in their structure. In the next section, we discuss the most used materials in the synthesis of NIR QDs and the properties obtained using different synthetic methodologies. Cd(II)-based QDs have been the most studied and applied material for QDs. The synthesis of Cd(II)-based QDs is one of the easiest from a synthetic point of view. The core precursors, which can vary from CdO (Peng and Peng, 2001Peng Z.A. Peng X. Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor.J. Am. Chem. Soc. 2001; 123: 183-184Crossref PubMed Scopus (2578) Google Scholar) to CdCl2 (Cui et al., 2018Cui Y. Zhang C. Song L. Yang J. Hu Z. Liu X. Facile synthesis of near-infrared emissive cds quantum dots for live cells imaging.J. Nanosci. Nanotechnol. 2018; 18: 2271-2277Crossref PubMed Google Scholar), are injected into a hot solution of the solvents and stirred for 30 to 60 min to yield NIR-emitting QDs (Figure 3). The solvent used in the synthesis has changed over the years from predominantly phosphine-based ones (tetradecylphosphonic acid, octadecylphosphonic acid) to fatty acids (oleic acid, 1-octadece) (Peng and Peng, 2001Peng Z.A. Peng X. Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor.J. Am. Chem. Soc. 2001; 123: 183-184Crossref PubMed Scopus (2578) Google Scholar). This change was driven by a necessity to adhere to a more environmentally friendly chemistry, and at the same time, fatty acids also allow for an easier transition between crystal shapes. This change in shape allows for the appearance of higher energy transitions in the crystal structure of the QDs, thus extending the emission toward longer wavelengths. Cd(II)-based NIR QDs that have been published in recent years comprise CdS (d = 3.7 nm, λex = 468 nm, λem = 730 nm), CdTe (d = 5.7 nm, λex = 576 nm, λem = 606 nm), and some alloyed compounds as well (Cui et al., 2018Cui Y. Zhang C. Song L. Yang J. Hu Z. Liu X. Facile synthesis of near-infrared emissive cds quantum dots for live cells imaging.J. Nanosci. Nanotechnol. 2018; 18: 2271-2277Crossref PubMed Google Scholar; Saikia et al., 2017Saikia D. Chakravarty S. Sarma N. Bhattacharjee S. Datta P. Adhikary N. Aqueous synthesis of highly stable cdte/zns core/shell quantum dots for bioimaging.Luminescence. 2017; 32: 401-408Crossref PubMed Scopus (12)" @default.
• W3132295265 created "2021-03-01" @default.
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• W3132295265 title "NIR-quantum dots in biomedical imaging and their future" @default.
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