Cell Microenvironment pH Sensing in 3D Microgels Using Fluorescent Carbon Dots

Microgel-based modular 3D in vitro microfluidic cell culture platforms

The combination of 3D in vitro cell culture and microfluidic technology has emerged as a powerful approach in biomedical engineering. It offers a more physiologically relevant model compared to traditional 2D cell cultures by allowing the assembly of micro-sized cellular structures, known as microgels. These microgels can be prepared and fabricated to mimic the in vivo characteristics of an ECM. We report here an economical and feasible microfluidic 3D in vitro culture platform that offers real-time monitoring of cellular proliferation by encapsulating pH-sensing carbon dots (CDs) with cells in the microgels. These CDs were shown to effectively evaluate proliferation within cell-encapsulated microgels in comparison with the traditional Alamar blue assay. The biggest advantage of this platform is its ability to co-culture different cell types, achieved by encapsulating the cells within individual microgels, spatially separating them while maintaining close proximity. In this modular system, each microgel acts as a unit of a specific cell type, allowing easy retrieval of cells and control over cell densities. We established the efficacy of this concept by co-culturing Huh-7 and NIH-3T3 cells within different microgel combinations, under both static and dynamic flow conditions. The heterotypic interactions were explored by assessing the functionality using albumin assay and CYP3A4 gene expression studies, along with performing drug toxicity assays. The functionality studies confirmed results from existing literature studies by showing an improved hepatic function in the presence of NIH-3T3, even in the dynamic state. This platform can be expanded to include multiple cell types, creating a complex tissue-like effect without requiring spatial patterning techniques.

Zwitterionic-hydrogel-based sensing system enables real-time ROS monitoring for ultra-long hypothermic cell preservation

Hypothermic preservation (HP) is highly desired for the maintenance of the viability of living cell specimens, e.g. rare cells in whole-blood samples or therapeutic cells, in an unfrozen state. However, the extension of the viable preservation time is a challenge because of the multiple injuries suffered by hypothermically preserved cells. Here, based on a dynamic bond crosslinked zwitterionic hydrogel, we established a sensing preservation system that could monitor the levels of reactive oxygen species (ROS) via real-time electronic signals and intelligent control of antioxidant addition, to completely prevent an excess of ROS in the whole-cell specimen. Furthermore, the hydrogel-based system can counter the extracellular-matrix-loss-induced anoikis of living cells. Based on the design aimed at affording protection against two primary HP injuries (i.e. ROS overproduction and anoikis) to cells, this system extended the preservation time of cell specimens under refrigerated conditions to 24 days. After preservation, the use of a mild cell retrieval process guaranteed the activity of the preserved living cells. This work not only possesses the potential to facilitate intelligent cell-based clinical applications, but also paves the way for the preparation of living materials that can host programmed cells with long-term survival. STATEMENT OF SIGNIFICANCE: An intelligent system based on a zwitterionic sensing hydrogel is established, which can afford ultra-long hypothermic cell-preservation times of up to 24 days. The system enables the real-time monitoring of ROS overproduction and intelligent antioxidant addition, because of the merging of the smart hydrogel with a computer intelligent detection and control system. Furthermore, the automatic addition of an antioxidant according to the ROS-signal changes produced by the ZBA hydrogel effectively prevented HP lesions, including ROS over-production and ECM loss, in the preserved living cells. Subsequently, the system could also be gently dissociated, to retrieve the preserved cells. This work provides a solution for the real-time monitoring and long-term HP of living specimens, which holds the promise of benefiting cell-based medicine and the development of genetically programmed cell-based living materials.

Metal-doped carbon dots for biomedical applications: From design to implementation

Carbon dots (CDs), as a new kind of fluorescent nanomaterials, show great potential for application in several fields due to their unique nano-size effect, easy surface functionalization, controllable photoluminescence, and excellent biocompatibility. Conventional preparation methods for CDs typically involve top-down and bottom-up approaches. Doping is a major step forward in CDs design methodology. Chemical doping includes both non-metal and metal doping, in which non-metal doping is an effective strategy for modulating the fluorescence properties of CDs and improving photocatalytic performance in several areas. In recent years, Metal-doped CDs have aroused the interest of academics as a promising nano-doping technique. This approach has led to improvements in the physicochemical and optical properties of CDs by altering their electron density distribution and bandgap capacity. Additionally, the issues of metal toxicity and utilization have been addressed to a large extent. In this review, we categorize metals into two major groups: transition group metals and rare-earth group metals, and an overview of recent advances in biomedical applications of these two categories, respectively. Meanwhile, the prospects and the challenges of metal-doped CDs for biomedical applications are reviewed and concluded. The aim of this paper is to break through the existing deficiencies of metal-doped CDs and fully exploit their potential. I believe that this review will broaden the insight into the synthesis and biomedical applications of metal-doped CDs.

Rapid Electrochemical Detection of Bacterial Sepsis in Cirrhotic Patients: A Microscaffold-Based Approach for Early Intervention

Sepsis is a dysregulated inflammatory response leading to multiple organ failure. Current methods of sepsis detection are time-consuming, involving nonspecific clinical signs, biomarkers, and blood cultures. Hence, efficient and rapid sepsis detection platforms are of utmost need for immediate antibiotic treatment. In the current study, a noninvasive rapid monitoring electrochemical sensing (ECS) platform was developed for the detection and classification of plasma samples of patients with liver cirrhosis by measuring the current peak shifts using the cyclic voltammetry (CV) technique. A total of 61 hospitalized cirrhotic patients with confirmed (culture-positive) or suspected (culture-negative) sepsis were enrolled. The presence of bacteria in the plasma was observed by growth kinetics, and for rapidness, the samples were co-encapsulated in microscaffolds with carbon nanodots that were sensitive enough to detect redox changes occurring due to the change in the pH of the surrounding medium, causing shifts in current peaks in the voltammograms within 2 h. The percentage area under the curve for confirmed infections was 94 and that with suspected cases was 87 in comparison to 69 and 71 with PCT, respectively. Furthermore, the charge was measured for class identification. The charge for LPS-absent bacteria ranged from -400 to -600 μC, whereas the charge for LPS-containing bacteria class ranged from -290 to -300 μC. Thus, the developed cost-effective system was sensitive enough to detect and identify bacterial sepsis.

Dual-Sensitive Fluorescent Nanoprobes for Simultaneously Monitoring In Situ Changes in pH and Matrix Metalloproteinase Expression in Stiffness-Tunable Three-Dimensional In Vitro Scaffolds

A tumor microenvironment often presents altered physicochemical characteristics of the extracellular matrix (ECM) including changes in matrix composition, stiffness, protein expression, pH, temperature, or the presence of certain stromal and immune cells. Of these, overexpression of matrix metalloproteinases (MMPs) and extracellular acidosis are the two major hallmarks of cancer that can be exploited for tumor detection. The change in matrix stiffness and the release of certain cytokines (TNF-α) in the tumor microenvironment play major roles in inducing MMP-9 expression in cancerous cells. This study highlights the role of mechanical cues in upregulating MMP-9 expression in cancerous cells using stiffness-tunable matrix compositions and dual-sensitive fluorescent nanoprobes. Ionically cross-linked 3D alginate/gelatin (AG) scaffolds with three stiffnesses were chosen to reflect the ECM stiffnesses corresponding to healthy and pathological tissues. Moreover, a dual-sensitive nanoprobe, an MMP-sensitive peptide conjugated to carbon nanoparticles with intrinsic pH fluorescence properties, was utilized for in situ monitoring of the two cancer hallmarks in the 3D scaffolds. This platform was further utilized for designing a 3D core-shell platform for spatially mapping tumor margins and for visualizing TNF-α-induced MMP-9 expression in cancerous cells.

Ubiquitin specific peptidase (USP37) mediated effects in microscaffold-encapsulated cells: a comprehensive study on growth, proliferation and EMT

Though significant advances have been made in developing therapeutic strategies for cancer, suitable in vitro models for mechanistically identifying relevant drug targets and understanding disease progression are still lacking. Most studies are generally performed using two-dimensional (2D) models, since these models can be readily established and allow high throughput assays. However, these models have also been reported as the reason for unreliable pre-clinical information. To avoid this discrepancy, three-dimensional (3D) cell culture models have been established and have demonstrated the potential to provide alternative ways to study tissue behavior. However, most of these models first require optimization and cell cultures with a certain density, thus adding a prepping step in the platform before it can be used for any studies. This limits their use in studies where the fundamental understanding of biological processes must be carried out in a short time frame. In this study, we developed a 3D cell culture system that tests a less explored cancer therapeutic target-the deubiquitinating enzyme ubiquitin specific peptidase 37 (USP37)-in different cancer cell lines using sensitive carbon dot pH nanosensors, which provides a rapid model for studies compared to the parallel model available commercially. This enzyme is found to be elevated in different cancers and has been reported to play a role in cell cycle regulation, oncogenesis and metastasis. However, the confirmation of the role of USP37 downregulation in cellular proliferation via appropriate in vitro 3D models has not been demonstrated. To establish the applicability of the developed 3D platform in studying such oncogenes, classical 2D models have been used in this study for identifying the role of USP37 in tumor progression and metastasis. The data clearly suggests that this ingeniously developed 3D cell culture system is a better alternative to 2D models to study the growth and migration of different cancer cell lines on depletion of oncogenic proteins like USP37 and its effect on epithelial-mesenchymal transition (EMT) markers, and it can further be targeted as a viable therapeutic option.

Biomass-Derived Carbon Dots as Nanoprobes for Smartphone-Paper-Based Assay of Iron and Bioimaging Application

A facile one-step carbonization approach is reported herein for the sustainable hydrothermal synthesis of fluorescent blue nitrogen-doped carbon quantum dots (NCQDs) using banana petioles obtained as biomass waste. These NCQDs were used to design a "turn-off" fluorescent probe, which exhibited excellent sensing capability toward the selective detection of micronutrient, Fe3+ ion, with a limit of detection (LOD) of 0.21 nM. The turn-off process involves the formation of a nonradiative charge transfer complex via a photoinduced electron transfer process. The sensor showed a linear range from 5 to 200 nM and was used for the estimation of Fe3+ ions in real plant samples. Further, a paper-based assay was developed for the quantitative estimation of Fe3+ with LOD values of 0.47 nM for solution-based assay and 0.94 nM for paper-based assay using a smartphone-based readout for potential on-field applications in precision agriculture. Bioimaging studies on banana leaf cells using NCQDs revealed the selective staining of stomata openings on leaf lamella. Therefore, this work provides a way for the valorization of biomass waste into functional nanomaterials without using any extra chemicals.

Preparation of Multicolor Fluorescent Carbon Dots Based on Catechol and o-Phthalaldehyde

As the foremost category of carbon materials, carbon dots (CDs) have been extensively applied in many domains because of their special fluorescence features and outstanding biocompatibility. However, in early studies of fluorescent CDs, as the fluorescence wavelength of most CDs was restricted to the blue or green region and was excitation dependent, the application of CDs was limited. In this study, three representative CDs, fluorescing yellow, green, and blue, were synthesized under alkaline, neutral, and acidic circumstances, respectively, while using a hydrothermal method in which catechol and phthalaldehyde acted as carbon sources and methanol functioned as the reaction solvent. The carbon nuclei of the three fluorescent CDs all had comparable graphite structures. The diversity of photoluminescence (PL) emission from these three CDs was attributed mainly to the different sizes of the sp² conjugated structures among them. Mixing synthesized CDs with epoxy resin, three colors (yellow, green, and blue) of LED using CIE coordinates (0.40, 0.44), (0.33, 0.46), and (0.21, 0.22), respectively, were successfully prepared.

Nanocomposite Hydrogels as Functional Extracellular Matrices

Over recent years, nano-engineered materials have become an important component of artificial extracellular matrices. On one hand, these materials enable static enhancement of the bulk properties of cell scaffolds, for instance, they can alter mechanical properties or electrical conductivity, in order to better mimic the in vivo cell environment. Yet, many nanomaterials also exhibit dynamic, remotely tunable optical, electrical, magnetic, or acoustic properties, and therefore, can be used to non-invasively deliver localized, dynamic stimuli to cells cultured in artificial ECMs in three dimensions. Vice versa, the same, functional nanomaterials, can also report changing environmental conditions-whether or not, as a result of a dynamically applied stimulus-and as such provide means for wireless, long-term monitoring of the cell status inside the culture. In this review article, we present an overview of the technological advances regarding the incorporation of functional nanomaterials in artificial extracellular matrices, highlighting both passive and dynamically tunable nano-engineered components.

Microfluidic Fabrication and Applications of Microgels and Hybrid Microgels

Smart microgels have gained much attention because of their wide range of applications in the field of biomedical, environmental, nanotechnological and catalysis sciences. Most of the applications of microgels are strongly affected by their morphology, size and size distribution. Various methodologies have been adopted to obtain polymer microgel particles. Droplet microfluidic techniques have been widely reported for the fabrication of highly monodisperse microgel particles to be used for various applications. Monodisperse microgel particles of required size and morphology can be achieved via droplet microfluidic techniques by simple polymerization of monomers in the presence of suitable crosslinker or by gelation of high molecular weight polymers. This report gives recent research progress in fabrication, characterization, properties and applications of microgel particles synthesized by microfluidic methods.

Microgels based on 0D-3D carbon materials: Synthetic techniques, properties, applications, and challenges

Microgels are three-dimensional (3D) colloidal hydrogel particles with outstanding features such as biocompatibility, good mechanical properties, tunable sizes from submicrometer to tens of nanometers, and large surface areas. Because of these unique qualities, microgels have been widely used in various applications. Carbon-based materials (CMs) with various dimensions (0-3D) have recently been investigated as promising candidates for the design and fabrication of microgels because of their large surface area, excellent conductivity, unique chemical stability, and low cost. Here, we provide a critical review of the specific characteristics of CMs that are being incorporated into microgels, as well as the state-of-the art applications of CM-microgels in pollutant adsorption and photodegradation, H₂ evoluation, CO₂ capture, soil conditioners, water retention, drug delivery, cell encapsulation, and tissue engineering. Advanced preparation techniques for CM-microgel systems are also summarized and discussed. Finally, challenges related to the low colloidal stability of CM-microgels and development strategies are examined. This review shows that CM-microgels have the potential to be widely used in various practical applications.

Dual-sensitive fluorescent nanoprobes for detection of matrix metalloproteinases and low pH in a 3D tumor microenvironment

The overexpression of matrix metalloproteinases and low extracellular pH are two key physiological parameters involved in cancer initiation, progression, and metastasis. These have been the targets for several cancer detection and imaging modalities. Here, dual-sensitive nanoprobes have been fabricated from carbon nanoparticles decorated with a MMP-9 sensitive peptide sequence. Carbon nanoparticles are known for their intrinsic fluorescence properties and hence used as a pH-sensing moiety in the nanoprobes. In addition to this, selective-cleavage of the peptide sequence by MMP-9 results in the generation of a fluorescence signal due to separation of the quencher molecule from the fluorophore attached onto the MMP-9 sensitive peptide sequence, resulting in its detection. This protease-specific activation of the nanoprobes helps in precise tumor environment detection and imaging. The nanoprobes were thoroughly characterized for their chemical, physical and biological activities. The potential of these dual-sensitive nanoprobes to distinguish tumor-like microenvironments (low pH and elevated MMP-9 levels) from non-cancerous ones was evaluated in vitro in 2D cell culture as well as in 3D microscaffolds. The fluorescence microscopy images obtained in both in vitro systems revealed that low pH and high MMP-9 levels could be successfully visualised using these dual-sensitive nanoprobes. Therefore, these nanoprobes would find potential applications as a non-invasive imaging tool for visualising tumor margins in real-time.

Advancements in Three Dimensional In-Vitro Cell Culture Models

The scientific field is observing a gradual shift from monolayer cultures to three-dimensional (3D) models, as they give a more relevant data in pre-clinical stages. This review summarizes the major techniques and materials used to develop 3D platforms, especially for cancer. It also discusses the challenges and some unresolved issues of the field and highlights some techniques that have made it to the market.

Biosensors to Monitor Cell Activity in 3D Hydrogel-Based Tissue Models

Three-dimensional (3D) culture models have gained relevant interest in tissue engineering and drug discovery owing to their suitability to reproduce in vitro some key aspects of human tissues and to provide predictive information for in vivo tests. In this context, the use of hydrogels as artificial extracellular matrices is of paramount relevance, since they allow closer recapitulation of (patho)physiological features of human tissues. However, most of the analyses aimed at characterizing these models are based on time-consuming and endpoint assays, which can provide only static and limited data on cellular behavior. On the other hand, biosensing systems could be adopted to measure on-line cellular activity, as currently performed in bi-dimensional, i.e., monolayer, cell culture systems; however, their translation and integration within 3D hydrogel-based systems is not straight forward, due to the geometry and materials properties of these advanced cell culturing approaches. Therefore, researchers have adopted different strategies, through the development of biochemical, electrochemical and optical sensors, but challenges still remain in employing these devices. In this review, after examining recent advances in adapting existing biosensors from traditional cell monolayers to polymeric 3D cells cultures, we will focus on novel designs and outcomes of a range of biosensors specifically developed to provide real-time analysis of hydrogel-based cultures.

Synthesis and Luminescent Properties of Carbon Nanodots Dispersed in Nanostructured Silicas

Luminescent carbon nanoparticles are a relatively new class of luminescent materials that have attracted the increasing interest of chemists, physicists, biologists and engineers. The present review has a particular focus on the synthesis and luminescent properties of carbon nanoparticles dispersed inside nanostructured silica of different natures: oxidized porous silicon, amorphous thin films, nanopowders, and nanoporous sol-gel-derived ceramics. The correlations of processing conditions with emission/excitation spectral properties, relaxation kinetics, and photoluminescence photodegradation behaviors are analyzed. Following the evolution of the photoluminescence (PL) through the "from-bottom-to-up" synthesis procedure, the transformation of molecular-like ultraviolet emission of organic precursor into visible emission of carbon nanoparticles is demonstrated. At the end of the review, a novel method for the synthesis of luminescent and transparent composites, in form of nanoporous silica filled with luminescent carbon nanodots, is presented. A prototype of white light emitting devices, constructed on the basis of such luminophores and violet light emitting diodes, is demonstrated.

A Review of Fluorescent Carbon Dots, Their Synthesis, Physical and Chemical Characteristics, and Applications

Carbon dots (CDs) are a particularly useful type of fluorescent nanoparticle that demonstrate biocompatibility, resistance to photobleaching, as well as diversity in composition and characteristics amongst the different types available. There are two main morphologies of CDs: Disk-shaped with 1-3 stacked sheets of aromatic carbon rings and quasi-spherical with a core-shell arrangement having crystalline and amorphous properties. They can be synthesized from various potentially environmentally friendly methods including hydrothermal carbonization, microwaving, pyrolysis or combustion, and are then purified via one or more methods. CDs can have either excitation wavelength-dependent or -independent emission with each having their own benefits in microscopic fluorescent imaging. Some CDs have an affinity for a particular cell type, organelle or chemical. This property allows the CDs to be used as sensors in a biological environment and can even provide quantitative information if the quenching or intensity of their fluorescence is dependent on the concentration of the analyte. In addition to fluorescent imaging, CDs can also be used for other applications including drug delivery, quality control, photodynamic therapy, and photocatalysis.

Carbon Dots: An Emerging Smart Material for Analytical Applications

Carbon dots (CDs) are optically active carbon-based nanomaterials. These nanomaterials can change their light emission properties in response to various external stimuli such as pH, temperature, pressure, and light. The CD's remarkable stimuli-responsive smart material properties have recently stimulated massive research interest for their exploitation to develop various sensor platforms. Herein, an effort has been made to review the major advances made on CDs, focusing mainly on its smart material attributes and linked applications. Since the CD's material properties are largely linked to their synthesis approaches, various synthesis methods, including surface passivation and functionalization of CDs and the mechanisms reported so far in their photophysical properties, are also delineated in this review. Finally, the challenges of using CDs and the scope for their further improvement as an optical signal transducer to expand their application horizon for developing analytical platforms have been discussed.

Orange emissive carbon nanodots for fluorescent and colorimetric bimodal discrimination of Cu2+ and pH

We have facilely synthesized orange emissive carbon nanodots (O-CDs) via a hydrothermal method using citric acid and 5-aminosalicylic acid.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

Probing the pH Microenvironment of Mesenchymal Stromal Cell Cultures on Additive-Manufactured Scaffolds

Despite numerous advances in the field of tissue engineering and regenerative medicine, monitoring the formation of tissue regeneration and its metabolic variations during culture is still a challenge and mostly limited to bulk volumetric assays. Here, a simple method of adding capsules-based optical sensors in cell-seeded 3D scaffolds is presented and the potential of these sensors to monitor the pH changes in space and time during cell growth is demonstrated. It is shown that the pH decreased over time in the 3D scaffolds, with a more prominent decrease at the edges of the scaffolds. Moreover, the pH change is higher in 3D scaffolds compared to monolayered 2D cell cultures. The results suggest that this system, composed by capsules-based optical sensors and 3D scaffolds with predefined geometry and pore architecture network, can be a suitable platform for monitoring pH variations during 3D cell growth and tissue formation. This is particularly relevant for the investigation of 3D cellular microenvironment alterations occurring both during physiological processes, such as tissue regeneration, and pathological processes, such as cancer evolution.

Renal Clearable Gold Nanoparticle-Functionalized Silk Film for in vivo Fluorescent Temperature Mapping

Implantable optical sensing devices that can continuously monitor physiological temperature changes hold great potential toward applications in healthcare and medical field. Here, we present a conceptual foundation for the design of biocompatible temperature sensing device by integrating renal clearable luminescent gold nanoparticles (AuNPs) with silk film (AuNPs-SF). We found that the AuNPs display strong temperature dependence in both near-IR fluorescence intensity and lifetime over a large temperature range (10-60°C), with a fluorescence intensity sensitivity of 1.72%/°C and lifetime sensitivity of 0.09 μs/°C. When integrated, the AuNPs with biocompatible silk film are implanted in the dorsal region of mice. The fluorescence imaging of the AuNPs-SF in the body shows a linear relationship between the average fluorescence intensity and temperature. More importantly, <3.68% ID gold are left in the body, and no adverse effect is observed for 8 weeks. This AuNPs-SF can be potentially used as a flexible, biocompatible, and implantable sensing device for in vivo temperature mapping.

Carbon Dots for Intracellular pH Sensing with Fluorescence Lifetime Imaging Microscopy

The monitoring of intracellular pH is of great importance for understanding intracellular trafficking and functions. It has various limitations for biosensing based on the fluorescence intensity or spectra study. In this research, pH-sensitive carbon dots (CDs) were employed for intracellular pH sensing with fluorescence lifetime imaging microscopy (FLIM) for the first time. FLIM is a highly sensitive method that is used to detect a microenvironment and it can overcome the limitations of biosensing methods based on fluorescence intensity. The different groups on the CDs surfaces changing with pH environments led to different fluorescence lifetime values. The CDs aqueous solution had a gradual change from 1.6 ns to 3.7 ns in the fluorescence lifetime with a pH range of 2.6-8.6. Similar fluorescence lifetime changes were found in pH buffer-treated living cells. The detection of lysosomes, cytoplasm, and nuclei in living cells was achieved by measuring the fluorescence lifetime of CDs. In particular, a phasor FLIM analysis was used to improve the pH imaging. Moreover, the effects of the coenzymes, amino acids, and proteins on the fluorescence lifetime of CDs were examined in order to mimic the complex microenvironment inside the cells.

Polysaccharide enabled biogenic fabrication of pH sensing fluorescent gold nanoclusters as a biocompatible tumor imaging probe

A biocompatible natural polysaccharide (PSP001) isolated from the fruit rind of Punica granatum was conjugated with L-cysteine (Y) to be used as a skeleton for the fabrication of fluorescent gold nanoclusters (AuNCs) represented as PSP-Y-AuNCs. With an average size of ~ 6 nm, PSP-Y-AuNCs demonstrated high quantum yield (31%), with a pH-sensitive fluorescence emission behavior. An emission maximum of 520 nm was obtained at acidic pH, which was blue shifted with increasing pH. This feature provides the possibilities for accurate ratiometric pH imaging. The PSP-Y-AuNCs not only demonstrated excellent biocompatibility with cancer cells and isolated peripheral lymphocytes and red blood cells but also demonstrated to be an active molecular imaging probe with appealing cellular uptake efficiency. The investigations with BALB/c mice further confirmed the non-toxic nature and in vivo imaging potential of the AuNCs. Estimation of the bio-distribution on solid tumor bearing syngeneic murine models revealed a tumor-targeted enhanced fluorescence emission pattern which is attributed to the pH responsive fluorescence behavior and the acidic microenvironment of the tumor. These findings were further confirmed with an impressive tumor accumulation pattern displayed in a xenograft of human cancer bearing nude mice. On account of their impressive biocompatibility and photophysical features, PSP-Y-AuNCs can exploited for the real-time fluorescence imaging of cancer tissues. Graphical abstract Fluorescent gold nanoclusters (PSP-Y-AuNCs) fabricated using a non-toxic natural polysaccharide (PSP001) demonstrated pH sensitive fluorescence emission pattern. The increased fluorescence readouts at acidic conditions and excellent biocompatibility made the PSP-Y-AuNCs an appealing candidate for in vivo tumor imaging applications.

Recent Progress in Electrochemical pH-Sensing Materials and Configurations for Biomedical Applications

pH-sensing materials and configurations are rapidly evolving toward exciting new applications, especially those in biomedical applications. In this review, we highlight rapid progress in electrochemical pH sensors over the past decade (2008-2018) with an emphasis on key considerations, such as materials selection, system configurations, and testing protocols. In addition to recent progress in optical pH sensors, our main focus in this review is on electromechanical pH sensors due to their significant advances, especially in biomedical applications. We summarize developments of electrochemical pH sensors that by virtue of their optimized material chemistries (from metal oxides to polymers) and geometrical features (from thin films to quantum dots) enable their adoption in biomedical applications. We further present an overview of necessary sensing standards and protocols. Standards ensure the establishment of consistent protocols, facilitating collective understanding of results and building on the current state. Furthermore, they enable objective benchmarking of various pH-sensing reports, materials, and systems, which is critical for the overall progression and development of the field. Additionally, we list critical issues in recent literary reporting and suggest various methods for objective benchmarking. pH regulation in the human body and state-of-the-art pH sensors (from ex vivo to in vivo) are compared for suitability in biomedical applications. We conclude our review by (i) identifying challenges that need to be overcome in electrochemical pH sensing and (ii) providing an outlook on future research along with insights, in which the integration of various pH sensors with advanced electronics can provide a new platform for the development of novel technologies for disease diagnostics and prevention.

Excitation-independent hollow orange-fluorescent carbon nanoparticles for pH sensing in aqueous solution and living cells

We report an ingenious strategy for fabrication of hollow orange fluorescent carbon nanoparticles (HFCNs) and demonstrate their applications for pH biosensing and fingerprint detection. HFCNs have been synthesized using 5-amino salicylic acid as carbon source via one-step hydrothermal treatment without further surface passivation or modification. The as-prepared HFCNs possess excellent hollow structure and bright orange fluorescence. The HFCNs display a remarkable emission enhancement in the orange fluorescence region when the pH is increased from 3.0 to 10.0. The pKa value of HFCNs is found to be 5.97 and a good linearity is shown in the pH range of 5.25-6.75, which makes HFCNs an effective intracellular pH imaging agent for acidic microenvironments. The confocal fluorescent microscopic images of HFCNs-stained latent fingerprint are achieved successfully, suggesting that they have great promise for practical criminal investigations as a simple, fast, and accurate tool.

Carbon-based hybrid nanogels: a synergistic nanoplatform for combined biosensing, bioimaging, and responsive drug delivery

Nanosized crosslinked polymer networks, named as nanogels, are playing an increasingly important role in a diverse range of applications by virtue of their porous structures, large surface area, good biocompatibility and responsiveness to internal and/or external chemico-physical stimuli. Recently, a variety of carbon nanomaterials, such as carbon quantum dots, graphene/graphene oxide nanosheets, fullerenes, carbon nanotubes, and nanodiamonds, have been embedded into responsive polymer nanogels, in order to integrate the unique electro-optical properties of carbon nanomaterials with the merits of nanogels into a single hybrid nanogel system for improvement of their applications in nanomedicine. A vast number of studies have been pursued to explore the applications of carbon-based hybrid nanogels in biomedical areas for biosensing, bioimaging, and smart drug carriers with combinatorial therapies and/or theranostic ability. New synthetic methods and structures have been developed to prepare carbon-based hybrid nanogels with versatile properties and functions. In this review, we summarize the latest developments and applications and address the future perspectives of these carbon-based hybrid nanogels in the biomedical field.

Bacterial growth sensing in microgels using pH-dependent fluorescence emission

Fast sensing of bacterial growth and antibiotic resistance by observing pH variation in microgels encapsulating bacteria and carbon dots.