Advances and Challenges of Fluorescent Nanomaterials for Synthesis and Biomedical Applications

With the rapid development of nanotechnology, new types of fluorescent nanomaterials (FNMs) have been springing up in the past two decades. The nanometer scale endows FNMs with unique optical properties which play a critical role in their applications in bioimaging and fluorescence-dependent detections. However, since low selectivity as well as low photoluminescence efficiency of fluorescent nanomaterials hinders their applications in imaging and detection to some extent, scientists are still in search of synthesizing new FNMs with better properties. In this review, a variety of fluorescent nanoparticles are summarized including semiconductor quantum dots, carbon dots, carbon nanoparticles, carbon nanotubes, graphene-based nanomaterials, noble metal nanoparticles, silica nanoparticles, phosphors and organic frameworks. We highlight the recent advances of the latest developments in the synthesis of FNMs and their applications in the biomedical field in recent years. Furthermore, the main theories, methods, and limitations of the synthesis and applications of FNMs have been reviewed and discussed. In addition, challenges in synthesis and biomedical applications are systematically summarized as well. The future directions and perspectives of FNMs in clinical applications are also presented.

Development of a reusable fluorescent nanosensor based on rhodamine B immobilized in Stöber silica for copper ion detection

This work has the goal of developing and evaluating a reusable fluorescent nanosensor for detection of Cu(ii) ion in aqueous solution, based on the immobilization of rhodamine B in silica nanoparticles prepared according to a modified Stöber method. In order to do this, a standard ammonium hydroxide ethanolic solution was mixed to ethanol under constant stirring, followed by the addition of tetraethoxysilane (TEOS). To immobilize the fluorescent reagent in the silica nanoparticles, rhodamine B ethanolic solution was added to the reacting mixture at different times (2; 3; 4 and 5 h) after starting the synthesis (which always lasts 7 h). The nanosensor obtained with the addition of rhodamine B after 5 h of synthesis showed the best sensitivity, measured as the fluorescence quenching, which was proportional to Cu(ii) ion. The nanosensor was selective to Cu(ii) ions and showed a linear range from 2.0 to 12.0 μmol L^-1, detection limit of 0.40 μmol L^-1, quantification limit of 1.3 μmol L^-1, response time of 50 s, being possible to be reused 3 times. The nanosensor was applied to the determination of Cu(ii) in sugar cane spirit and the results obtained did not show significant differences from those obtained by atomic absorption spectrometry at a confidence level of 95%.

The Role of 8-Amidoquinoline Derivatives as Fluorescent Probes for Zinc Ion Determination

Mass-spectrometry-based and X-ray fluorescence-based techniques have allowed the study of the distribution of Zn2+ ions at extracellular and intracellular levels over the past few years. However, there are some issues during purification steps, sample preparation, suitability for quantification, and the instruments' availability. Therefore, work on fluorescent sensors based on 8-aminoquinoline as tools to detect Zn2+ ions in environmental and biological applications has been popular. Introducing various carboxamide groups into an 8-aminoquinoline molecule to create 8-amidoquinoline derivatives to improve water solubility and cell membrane permeability is also a recent trend. This review aims to present a general overview of the fluorophore 8-aminoquinoline and its derivatives as Zn2+ receptors for zinc sensor probes. Various fluorescent chemosensor designs based on 8-amidoquinoline and their effectiveness and potential as a recognition probe for zinc analysis were discussed. Based on this review, it can be concluded that derivatives of 8-amidoquinoline have vast potential as functional receptors for zinc ions primarily because of their fast reactivity, good selectivity, and bio-compatibility, especially for biological applications. To better understand the Zn2+ ion fluorophores' function, diversity of the coordination complex and geometries need further studies. This review provides information in elucidating, designing, and exploring new 8-amidoquinoline derivatives for future studies for the improvement of chemosensors that are selective and sensitive to Zn2+.

A unique benzimidazole-naphthalene hybrid molecule for independent detection of Zn2+ and N3- ions: Experimental and theoretical investigations

Single crystal X-ray structurally characterized benzimidazole-naphthalene hybrid (NABI) functions as a unique dual analyte sensor that can detect Zn²⁺ cation and N₃^- anion independently. The NABI forms chelate with Zn²⁺ to inhibit internal charge transfer (ICT) and CHN isomerisation resulting chelation enhanced fluorescence (CHEF). On the other hand, the sensing of N₃^- is based on formation of supramolecular H-bonded rigid assembly. The association constant of NABI for Zn²⁺ and N₃^- ions are 19 × 10⁴ M^-1 and 11 × 10² M^-1, respectively. Corresponding limit of detections (LOD) are 6.85 × 10^-8 and 1.82 × 10^-7 M, respectively. NABI efficiently detects intracellular Zn²⁺ and N₃^- ions with no cytotoxicity on J774A.1cells under fluorescence microscope. DFT studies unlock underlying spectroscopic properties of free NABI and Zn²⁺/N₃^- bound forms.

Ratiometric optical nanoprobes enable accurate molecular detection and imaging

Exploring and understanding biological and pathological changes are of great significance for early diagnosis and therapy of diseases. Optical sensing and imaging approaches have experienced major progress in this field. Particularly, an emergence of various functional optical nanoprobes has provided enhanced sensitivity, specificity, targeting ability, as well as multiplexing and multimodal capabilities due to improvements in their intrinsic physicochemical and optical properties. However, one of the biggest challenges of conventional optical nanoprobes is their absolute intensity-dependent signal readout, which causes inaccurate sensing and imaging results due to the presence of various analyte-independent factors that can cause fluctuations in their absolute signal intensity. Ratiometric measurements provide built-in self-calibration for signal correction, enabling more sensitive and reliable detection. Optimizing nanoprobe designs with ratiometric strategies can surmount many of the limitations encountered by traditional optical nanoprobes. This review first elaborates upon existing optical nanoprobes that exploit ratiometric measurements for improved sensing and imaging, including fluorescence, surface enhanced Raman scattering (SERS), and photoacoustic nanoprobes. Next, a thorough discussion is provided on design strategies for these nanoprobes, and their potential biomedical applications for targeting specific biomolecule populations (e.g. cancer biomarkers and small molecules with physiological relevance), for imaging the tumor microenvironment (e.g. pH, reactive oxygen species, hypoxia, enzyme and metal ions), as well as for intraoperative image guidance of tumor-resection procedures.

Diverse Applications of Nanomedicine

The design and use of materials in the nanoscale size range for addressing medical and health-related issues continues to receive increasing interest. Research in nanomedicine spans a multitude of areas, including drug delivery, vaccine development, antibacterial, diagnosis and imaging tools, wearable devices, implants, high-throughput screening platforms, etc. using biological, nonbiological, biomimetic, or hybrid materials. Many of these developments are starting to be translated into viable clinical products. Here, we provide an overview of recent developments in nanomedicine and highlight the current challenges and upcoming opportunities for the field and translation to the clinic.

A highly selective ratiometric fluorescent probe for the cascade detection of Zn2+ and H2PO4− and its application in living cell imaging

A simple ratiometric sensor (L1) for the cascade detection of Zn²⁺ and H₂PO₄⁻ with high selectivity was reported based on the intermolecular charge transfer (ICT) mechanism.

Triazole-Linked Quinoline Conjugate of Glucopyranose: Selectivity Comparison among Zn2+, Cd2+, and Hg2+ Based on Spectroscopy, Thermodynamics, and Microscopy, and Reversible Sensing of Zn2+ and the Structure of the Complex Using DFT

A water-soluble triazole-linked quinoline conjugate of glucopyranose (L) has been synthesized and characterized, and its single-crystal X-ray diffraction (XRD) structure has been established. Binding of L toward different biologically relevant metal ions has been studied using fluorescence and absorption spectroscopy in HEPES buffer at pH 7.4. The conjugate L detects Zn2+ and Cd2+ with 30 ± 2 and 14 ± 1-fold fluorescence enhancement, respectively, but in the case of Hg2+, only a fluorescence quench was observed. The stoichiometry of the complex is 1:2 metal ion to the ligand in the case of Zn2+ and Cd2+ resulting in [Zn(L)2] and [Cd(L)2], and it is 1:1 in the case of Hg2+, as confirmed from their electrospray ionization mass spectrometry (ESIMS) spectra. Zn2+ shows greater exothermicity over Cd2+, whereas Hg2+ shows endothermicity , which supports the differences in their binding strength and the nature of the corresponding complex. L exhibits rod-shaped particles and upon complexation with Zn2+, it exhibits sphere-like morphological features in scanning electron microscopy (SEM) images. However, clustered aggregates are observed in Cd2+, whereas the [HgL] complex exhibits small fused spherical structures, and therefore the signature of these ions is seen in microscopy images. The computational studies revealed that the syn-[Zn(L)2] complex is stabilized by 9.7 kcal mol-1 more than that in the case of anti-[Zn(L)2] owing to the formation of hydrogen bonds between the two glucosyl moieties within the syn-complex. Among the anions studied, [Zn(L)2] is sensitive and selective toward the phosphate ion (H2PO4 -) with a minimum detection limit of 16 ± 2 ppb. Similarly, the [HgL] can act as a secondary sensor for CN- while also exhibiting reversibility. Based on the input-output characteristics, INHIBIT logic gate was built in the case of Zn2+ vs H2PO4 - and IMPLICATION logic gate was built in the case of Hg2+ vs CN-.

Discovery of a novel family of polycyclic aromatic molecules with unique reactivity and members valuable for fluorescent sensing and medicinal chemistry

We review major achievements in the study of derivatives of 1-oxo-1H-phenalene-2,3-dicarbonitrile.

A thiacalix[4]crown based chemosensor for Zn2+ and H2PO4−: sequential logic operations at the molecular level

A thiacalix[4]crown based di topic receptor 3 has been synthesized which shows negative allosteric behaviour between Zn²⁺/K⁺ ions. Further, 3-Zn²⁺ complex has been used for detection of H₂PO⁴⁻.

Development of organically modified silica nanoparticles for monitoring the intracellular level of oxygen using a frequency-domain FLIM platform

The dynamic quenching of luminescence derived from Ru(dpp₃)²⁺-doped ORMOSIL nanoparticles is used for monitoring of the intracellular oxygen concentration.

Silica nanoparticles for cell imaging and intracellular sensing

There is increasing interest in the use of nanoparticles (NPs) for biomedical applications. In particular, nanobiophotonic approaches using fluorescence offers the potential of high sensitivity and selectivity in applications such as cell imaging and intracellular sensing. In this review, we focus primarily on the use of fluorescent silica NPs for these applications and, in so doing, aim to enhance and complement the key recent review articles on these topics. We summarize the main synthetic approaches, namely the Stöber and microemulsion processes, and, in this context, we deal with issues in relation to both covalent and physical incorporation of different types of dyes in the particles. The important issue of NP functionalization for conjugation to biomolecules is discussed and strategies published in the recent literature are highlighted and evaluated. We cite recent examples of the use of fluorescent silica NPs for cell imaging in the areas of cancer, stem cell and infectious disease research, and we review the current literature on the use of silica NPs for intracellular sensing of oxygen, pH and ionic species. We include a short final section which seeks to identify the main challenges and obstacles in relation to the potential widespread use of these particles for in vivo diagnostics and therapeutics.

Substituent-dependent fluorescent sensors for zinc ions based on carboxamidoquinoline

A series of carboxamidoquinoline-based fluorescent sensors (the AQZ family) were synthesized and characterized. The AQZ family members were highly soluble in water and showed good selectivity for Zn(2+)via enhanced fluorescence in aqueous buffer solution. Fluorescence signals could be tuned from dual-wavelength ratiometric changes to changes in the intensity of a single wavelength upon binding Zn(2+) through the introduction of different substituents onto the quinoline ring. Concentrations of free Zn(2+) of 10(-5)-10(-6) M could be detected using the sensors. Changes of substituents and their positions on the quinoline ring influenced the sensitivity for Zn(2+), but had little effect on Zn(2+) affinities.