A Gold Nanoparticle Bio-Optical Transponder to Dynamically Monitor Intracellular pH

Recent advances in the design of intracellular pH sensing nanoprobes based on organic and inorganic materials

In the biological system, the intracellular pH (pHi) plays an important role in regulating diverse physiological activities, including enzymatic action, ion transport, cell proliferation, metabolism, and programmed cell death. The monitoring of pH inside living cells is also crucial for studying cellular events such as phagocytosis, endocytosis, and receptor-ligand internalization. Furthermore, some organelles, viz., endosomes and lysosomes, have intracompartmental pH, which is critical for maintaining the stability of protein structure and function. The dysfunction and abnormal pH regulation can result in terminal diseases such as cancer, Alzheimer, and so forth. Therefore, the accuracy of intracellular pH measurement is always the top priority and demands cutting-edge research and analysis. Such techniques, such as Raman spectroscopy and fluorescence imaging, preferably use nanotechnology due to their remarkable advantages, such as a non-invasive approach and providing accuracy, repeatability, and reproducibility. In the past decades, there have been numerous attempts to design and construct non-invasive organic and inorganic materials-based nanoprobes for pHi sensing. For Raman-based techniques, metal nanostructures such as Au/Ag/Cu nanoparticles are utilized to enhance the signal intensity. As for the fluorescence-based studies, the organic-based small molecules, such as dyes, show higher sensitivity toward pH. However, they possess several drawbacks, including high photobleaching rate, and autofluorescence background signals. To this end, there are alternative nanomaterials proposed, including semiconductor quantum dots (QDs), carbon QDs, upconversion nanoparticles, and so forth. Moreover, the fluorescence technique allows for ratiometric measurement of pHi, which as a result, offers a reliable calibration curve. This timely review will critically examine the current progression in the existing nanoprobes. In addition, based on our knowledge and available research findings, we provide a brief future outlook that may advance the state-of-the-art methodologies for pHi sensing.

Recent Advancements in the Design of Nanodelivery Systems of siRNA for Cancer Therapy

RNA interference (RNAi) has increased the possibility of restoring RNA drug targets for cancer treatment. Small interfering RNA (siRNA) is a promising therapeutic RNAi tool that targets the defective gene by inhibiting its mRNA expression and stopping its translation. However, siRNAs have flaws like poor intracellular trafficking, RNase degradation, rapid kidney filtration, off-targeting, and toxicity, which limit their therapeutic efficiency. Nanocarriers (NCs) have been designed to overcome such flaws and increase antitumor activity. Combining siRNA and anticancer drugs can give synergistic effects in cancer cells, making them a significant gene-modification tool in cancer therapy. Our discussion of NCs-mediated siRNA delivery in this review includes their mechanism, limitations, and advantages in comparison with naked siRNA delivery. We will also discuss organic NCs (polymers and lipids) and inorganic NCs (quantum dots, carbon nanotubes, and gold) that have been reported for extensive delivery of therapeutic siRNA to tumor sites. Finally, we will conclude by discussing the studies based on organic and inorganic NCs-mediated siRNA drug delivery systems conducted in the years 2020 and 2021.

Mechanistic Understanding of DNA Denaturation in Nanoscale Thermal Gradients Created by Femtosecond Excitation of Gold Nanoparticles

There is significant interest in developing photothermal systems that can precisely control the structure and function of biomolecules through local temperature modulation. One specific application is the denaturation of double-stranded (ds) DNA through femtosecond (fs) laser pulse optical heating of gold nanoparticles (AuNPs); however, the mechanism of DNA melting in these systems is not fully understood. Here, we utilize 55 nm AuNPs with surface-tethered dsDNA, which are locally heated using fs laser pulses to induce DNA melting. By varying the dsDNA distance from the AuNP surface and the laser pulse energy fluence, this system is used to study how the nanosecond duration temperature increase and the steep temperature gradient around the AuNP affect dsDNA dehybridization. Through modifying the distance between the dsDNA and AuNP surface by 3.8 nm in total and the pulse energy fluence from 7.1 to 14.1 J/m², the dehybridization rates ranged from 0.002 to 0.05 DNA per pulse, and the total amount of DNA released into solution was controlled over a range of 26-93% in only 100 s of irradiation. By shifting the dsDNA position as little as ∼1.1 nm, the average dsDNA dehybridization rate is altered up to 30 ± 2%, providing a high level of control over DNA melting and release. By comparing the theoretical temperature around the dsDNA to the experimentally derived temperature, we find that maximum or peak temperatures have a greater influence on the dehybridization rate when the dsDNA is closer to the AuNP surface and when lower laser pulse fluences are used. Furthermore, molecular dynamics simulations mimicking the photothermal heat pulse around a AuNP provide mechanistic insight into the stochastic nature of dehybridization and demonstrate increased base pair separation near the AuNP surface during laser pulse heating when compared to steady-state heating. Understanding how biological materials respond to the short-lived and non-uniform temperature increases innate to fs laser pulse optical heating of AuNPs is critical to improving the functionality and precision of this technique so that it may be implemented into more complex biological systems.

A Reversible Plasmonic Nanoprobe for Dynamic Imaging of Intracellular pH during Endocytosis

Understanding the pH evolution during endocytosis is essential for our comprehension of the fundamental processes of biology as well as effective nanotherapeutic design. Herein, we constructed a plasmonic Au@PANI core-shell...

pH-Triggered Aggregation of Gold Nanoparticles for Enhanced Labeling and Long-Term CT Imaging Tracking of Stem Cells in Pulmonary Fibrosis Treatment

Gold nanoparticles (AuNPs) pose a great challenge in the development of nanotracers that can self-adaptively alter their properties in response to certain cellular environments for long-term stem cell tracking. Herein, pH-sensitive Au nanotracers (CPP-PSD@Au) are fabricated by sequential coupling of AuNPs with sulfonamide-based polymer (PSD) and cell-penetrating peptide (CPP), which can be efficiently internalized by mesenchymal stem cells (MSCs) and undergo pH-induced self-assembly in endosomes, facilitating long-term computed tomography (CT) imaging tracking MSCs in a murine model of idiopathic pulmonary fibrosis (IPF). Using the CPP-PSD@Au, the transplanted MSCs for the first time can be monitored with CT imaging for up to 35 days after transplantation into the lung of IPF mice, clearly elucidating the migration process of MSCs in vivo. Moreover, we preliminarily explored the mechanism of the CPP-PSD@Au labeled MSCs in the alleviation of IPF, including recovery of alveolar integrity, decrease of collagen deposition, as well as down-regulation of relevant cytokine level. This work facilitates our understanding of the behavior and effect of MSCs in the therapy of IPF, thereby providing an important insight into the stem cell-based treatment of lung diseases.

Aptamer Conjugated Gold Nanostar-Based Distance-Dependent Nanoparticle Surface Energy Transfer Spectroscopy for Ultrasensitive Detection and Inactivation of Corona Virus

The ongoing outbreak of the coronavirus infection has killed more than 2 million people. Herein, we demonstrate that Rhodamine 6G (Rh-6G) dye conjugated DNA aptamer-attached gold nanostar (GNS)-based distance-dependent nanoparticle surface energy transfer (NSET) spectroscopy has the capability of rapid diagnosis of specific SARS-CoV-2 spike recombinant antigen or SARS-CoV-2 spike protein pseudotyped baculovirus within 10 min. Because Rh-6G-attached single-stand DNA aptamer wrapped the GNS, 99% dye fluorescence was quenched because of the NSET process. In the presence of spike antigen or virus, the fluorescence signal persists because of the aptamer-spike protein binding. Specifically, the limit of detection for the NSET assay has been determined to be 130 fg/mL for antigen and 8 particles/mL for virus. Finally, we have demonstrated that DNA aptamer-attached GNSs can stop virus infection by blocking the angiotensin-converting enzyme 2 (ACE2) receptor binding capability and destroying the lipid membrane of the virus.

Tracking endocytosis and intracellular distribution of spherical nucleic acids with correlative single-cell imaging

A comprehensive understanding of interactions between nanoparticles (NPs) and biological components is critical to the clinical application of NPs and nanomedicine. Here we provide a step-by-step correlative imaging approach to investigate plasmonic NPs of different aggregation states at the single-cell level. Traceable spherical nucleic acids (SNAs) are fabricated by decorating 50-nm spherical gold NPs with fluorophore-labeled DNA, serving as dually emissive (fluorescent and plasmonic) NPs. The in situ correlative imaging with dark-field microscopy (DFM) and fluorescence microscopy (FM) reveals intracellular distribution of SNAs, whereas DFM combined with scanning electron microscopy (SEM) allows semi-quantification of SNA clustering states in solution. The imaging data are analyzed by ImageJ and a colorimetry-based algorithm written in Python. The clustering states of SNAs in a single cell can be efficiently distinguished within 20 s. This method can be readily installed to monitor real-time endocytosis and cellular distribution of plasmonic NPs of different aggregation states and to quantitatively image targets of interest (e.g., specific DNA, messenger RNA, peptides or proteins) in living cells. The entire procedure can be completed in 3-5 d and requires standard DFM, FM and SEM imaging and data analysis skills and equipment.

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.

Live-Cell Surface-Enhanced Raman Spectroscopy Imaging of Intracellular pH: From Two Dimensions to Three Dimensions

Visualization of intracellular pH (i-pH) using surface-enhanced Raman spectroscopy (SERS) plays an important role toward understanding of cellular processes including their interactions with nanoparticles. However, conventional two-dimensional SERS imaging often fails to take into consideration changes occurring in the whole-cell volume. We therefore aimed at obtaining a comprehensive i-pH profile of living cells by means of three-dimensional (3D) SERS imaging, thereby visualizing dynamic i-pH distribution changes in a single cell. We devised here a biocompatible and highly stable SERS pH probe, comprising plasmonic gold nanostars functionalized with a pH-sensitive Raman reporter tag-4-mercaptobenzoic acid-and protected by a cationic biocompatible polymer, poly-l-arginine hydrochloride (PA). The positively charged PA coating plays a double role in enhancing cell uptake and providing chemical and colloidal stability in cellular environments. The SERS-active pH probe allowed visualization of local changes in i-pH, such as acidification during nanoparticle (NP) endocytosis. We provide evidence of i-pH changes during NP endocytosis via high-resolution 3D SERS imaging, thereby opening new avenues toward the application of SERS to intracellular studies.

Simultaneous Single-Particle Tracking and Dynamic pH Sensing Reveal Lysosome-Targetable Mesoporous Silica Nanoparticle Pathways

Nanoparticle (NP)-based targeted drug delivery is intended to transport therapeutically active molecules to specific cells and particular intracellular compartments. However, there is limited knowledge regarding the complete route of NPs in this targeting scenario. In this study, simultaneously performing motion and dynamic pH sensing using single-particle tracking (SPT) leads to an alternative method of gaining insights into the mesoporous silica nanoparticle's (MSN) journey in targeting lysosome. Two different pH-sensitive dyes and a reference dye are incorporated into mesoporous silica nanoparticles (MSNs) via co-condensation to broaden the measurable pH range (pH 4-7.5) of the nanoprobe. The phosphonate, amine, and lysosomal sorting peptides (YQRLGC) are conjugated onto the MSN's surface to study intracellular nano-biointeractions of two oppositely charged and lysosome-targetable MSNs. The brightness and stability of these MSNs allow their movement and dynamic pH evolution during their journey to be simultaneously monitored in real time. Importantly, a multidimensional analysis of MSN's movement and local pH has revealed new model intracellular dynamic states and distributions of MSNs, previously inaccessible when using single parameters alone. A key result is that YQRLGC-conjugated MSNs took an alternative route to target lysosomes apart from the traditional one, which sped up to 4 h and enhanced their targeting efficiency (up to 32%). The findings enrich our understanding of the intracellular journey of MSNs. This study offers complementary information on correlating the surface design with the full pathway of nanoparticles to achieve targeted delivery of therapeutic payload.

Femtosecond Laser Pulse Excitation of DNA-Labeled Gold Nanoparticles: Establishing a Quantitative Local Nanothermometer for Biological Applications

Femtosecond (fs) laser pulsed excitation of plasmonic nanoparticle (NP)-biomolecule conjugates is a promising method to locally heat biological materials. Studies have demonstrated that fs pulses of light can modulate the activity of DNA or proteins when attached to plasmonic NPs; however, the precision over subsequent biological function remains largely undetermined. Specifically, the temperature the localized biomolecules "experience" remains unknown. We used 55 nm gold nanoparticles (AuNPs) displaying double-stranded (ds) DNA to examine how, for dsDNA with different melting temperatures, the laser pulse energy fluence and bulk solution temperature affect the rate of local DNA denaturation. A universal "template" single-stranded DNA was attached to the AuNP surface, and three dye-labeled probe strands, distinct in length and melting temperature, were hybridized to it creating three individual dsDNA-AuNP bioconjugates. The dye-labeled probe strands were used to quantify the rate and amount of DNA release after a given number of light pulses, which was then correlated to the dsDNA denaturation temperature, resulting in a quantitative nanothermometer. The localized DNA denaturation rate could be modulated by more than threefold over the biologically relevant range of 8-53 °C by varying pulse energy fluence, DNA melting temperature, and surrounding bath temperature. With a modified dissociation equation tailored for this system, a "sensed" temperature parameter was extracted and compared to simulated AuNP temperature profiles. Determining actual biological responses in such systems can allow researchers to design precision nanoscale photothermal heating sources.

Quantitative Particle Uptake by Cells as Analyzed by Different Methods

There is a large number of two-dimensional static in vitro studies about the uptake of colloidal nano- and microparticles, which has been published in the last decade. In this Minireview, different methods used for such studies are summarized and critically discussed. Supplementary experimental data allow for a direct comparison of the different techniques. Emphasis is given on how quantitative parameters can be extracted from studies in which different experimental techniques have been used, with the goal of allowing better comparison.

The effect of phospho-peptide on the stability of gold nanoparticles and drug delivery

BACKGROUND

Gold nanoparticles (AuNPs) have been proposed for many applications in medicine and bioanalysis. For use in all these applications, maintaining the stability of AuNPs in solution by suppressing aggregation is paramount. Herein, the effects of amino acids were investigated in stabilizing AuNPs by rationally designed peptide scaffolds.

RESULTS

Compared to other tested amino acids, phosphotyrosine (pY) significantly stabilized AuNPs. Our results indicated that pY modified AuNPs presented a high level of stability in various solutions, and had good biocompatibility. When a pY-peptide was used in stabilizing AuNPs, the phosphate group could be removed by phosphatases, which subsequently caused the aggregation and the cargo release of AuNPs. In vitro study showed that AuNPs formed aggregation in a phosphatase concentration depending manner. The aggregation of AuNPs was well correlated with the enzymatic activity (R² = 0.994). In many types of cancer, a significant increase in phosphatases has been observed. Herein, we demonstrated that cancer cells treated with pY modified AuNPs in conjunction with doxorubicin killed SGC-7901 cells with high efficiency, indicating that the pY peptide stabilized AuNPs could be used as carriers for targeted drug delivery.

CONCLUSION

In summary, pY peptides can act to stabilize AuNPs in various solutions. In addition, the aggregation of pY-AuNPs could be tuned by phosphatase. These results provide a basis for pY-AuNPs acting as potential drug carriers and anticancer efficacy.

Bioconjugation strategy for cell surface labelling with gold nanostructures designed for highly localized pH measurement

Regulation of intracellular pH is critically important for many cellular functions. The quantification of proton extrusion in different types of cells and physiological conditions is pivotal to fully elucidate the mechanisms of pH homeostasis. Here we show the use of gold nanoparticles (AuNP) to create a high spatial resolution sensor for measuring extracellular pH in proximity of the cell membrane. We test the sensor on HepG2 liver cancer cells and MKN28 gastric cancer cells before and after inhibition of Na⁺/H⁺ exchanger. The gold surface conjugation strategy is conceived with a twofold purpose: i) to anchor the AuNP to the membrane proteins and ii) to quantify the local pH from AuNP using surface enhanced Raman spectroscopy (SERS). The nanometer size of the cell membrane anchored sensor and the use of SERS enable us to visualize highly localized variation of pH induced by H⁺ extrusion, which is particularly upregulated in cancer cells.

Understanding the pH changes at cell surfaces is important for understanding the mechanisms of different physiological processes. Here, the authors report on the development of a cell membrane anchored gold nanoparticle for surface enhanced Raman spectroscopy determination of pH.