Simultaneous intracellular redox potential and pH measurements in live cells using SERS nanosensors

High-Resolution and Dynamic Visualization of Intracellular Redox Potential Using a Metal-Organic Framework-Functionalized Nanopotentiometer

Redox potential plays a key role in regulating intracellular signaling pathways, with its quantitative analysis in individual cells benefiting our understanding of the underlying mechanism in the pathophysiological events. Here, a metal organic framework (MOF)-functionalized SERS nanopotentiometer has been developed for the dynamic monitoring of intracellular redox potential. The approach is based on the encapsulation of zirconium-based MOF (Uio-66-F4) on a surface of gold-silver nanorods (Au-Ag NRs) that is modified with the newly synthesized redox-sensitive probe ortho-mercaptohydroquinone (HQ). Thanks to size exclusion of MOF as the chemical protector, the nanopotentiometer can be adapted to long-term use and possess high anti-interference ability toward nonredox species. Combining the superior fingerprint identification of SERS with the electrochemical activity of the quinone/hydroquinone, the nanopotentiometer shows a reversible redox responsivity and can quantify redox potential with a relatively wide range of -250-100 mV. Furthermore, the nanopotentiometer allows for dynamic visualization of intracellular redox potential changes induced by drugs' stimulation in a high-resolution manner. The developed approach would be promising for offering new insights into the correlation between redox potential and tumor proliferation-involved processes such as oxidative stress and hypoxia.

SERS microsensors for pH measurements in the lumen and ECM of stem cell derived human airway organoids

Patient derived organoids have the potential to improve the physiological relevance of in vitro disease models. However, the 3D architecture of these self-assembled cellular structures makes probing their biochemistry more complex than in traditional 2D culture. We explore the application of surface enhanced Raman scattering microsensors (SERS-MS) to probe local pH gradients within patient derived airway organoid cultures. SERS-MS consist of solid polymer cores decorated with surface immobilised gold nanoparticles which are functionalised with pH sensitive reporter molecule 4-mercaptobenzoic acid (MBA). We demonstrate that by mixing SERS-MS into the extracellular matrix (ECM) of airway organoid cultures the probes can be engulfed by expanding organoids and report on local pH in the organoid lumen and ECM.

New Titanocene (IV) Dicarboxylates with Potential Cytotoxicity: Synthesis, Structure, Stability and Electrochemistry

The search for new anticancer drugs based on biogenic metals, which have weaker side effects compared to platinum-based drugs, remains an urgent task in medicinal chemistry. Titanocene dichloride, a coordination compound of fully biocompatible titanium, has failed in pre-clinical trials but continues to attract the attention of researchers as a structural framework for the development of new cytotoxic compounds. In this study, a series of titanocene (IV) carboxylate complexes, both new and those known from the literature, was synthesized, and their structures were confirmed by a complex of physicochemical methods and X-ray diffraction analysis (including one previously unknown structure based on perfluorinated benzoic acid). The comprehensive comparison of three approaches for the synthesis of titanocene derivatives known from the literature (the nucleophilic substitution of chloride anions of titanocene dichloride with sodium and silver salts of carboxylic acids as well as the reaction of dimethyltitanocene with carboxylic acids themselves) made it possible to optimize these methods to obtain higher yields of individual target compounds, generalize the advantages and disadvantages of these techniques, and determine the substrate frames of each method. The redox potentials of all obtained titanocene derivatives were determined by cyclic voltammetry. The relationship between the structure of ligands, the reduction potentials of titanocene (IV), and their relative stability in redox processes, as obtained in this work, can be used for the design and synthesis of new effective cytotoxic titanocene complexes. The study of the stability of the carboxylate-containing derivatives of titanocene obtained in the work in aqueous media showed that they were more resistant to hydrolysis than titanocene dichloride. Preliminary tests of the cytotoxicity of the synthesised titanocene dicarboxilates on MCF7 and MCF7-10A cell lines demonstrated an IC50 ≥ 100 μM for all the obtained compounds.

Distinct effects of intracellular vs. extracellular acidic pH on the cardiac metabolome during ischemia and reperfusion

Tissue ischemia results in intracellular pH (pHIN) acidification, and while metabolism is a known driver of acidic pHIN, less is known about how acidic pHIN regulates metabolism. Furthermore, acidic extracellular (pHEX) during early reperfusion confers cardioprotection, but how this impacts metabolism is unclear. Herein we employed LCMS based targeted metabolomics to analyze perfused mouse hearts exposed to: (i) control perfusion, (ii) hypoxia, (iii) ischemia, (iv) enforced acidic pHIN, (v) control reperfusion, and (vi) acidic pHEX (6.8) reperfusion. Surprisingly little overlap was seen between metabolic changes induced by hypoxia, ischemia, and acidic pHIN. Acidic pHIN elevated metabolites in the top half of glycolysis, and enhanced glutathione redox state. Meanwhile, acidic pHEX reperfusion induced substantial metabolic changes in addition to those seen in control reperfusion. This included elevated metabolites in the top half of glycolysis, prevention of purine nucleotide loss, and an enhancement in glutathione redox state. These data led to hypotheses regarding potential roles for methylglyoxal inhibiting the mitochondrial permeability transition pore, and for acidic inhibition of ecto-5'-nucleotidase, as potential mediators of cardioprotection by acidic pHEX reperfusion. However, neither hypothesis was supported by subsequent experiments. In contrast, analysis of cardiac effluents revealed complex effects of pHEX on metabolite transport, suggesting that mildly acidic pHEX may enhance succinate release during reperfusion. Overall, each intervention had distinct and overlapping metabolic effects, suggesting acidic pH is an independent metabolic regulator regardless which side of the cell membrane it is imposed.

Preserving DNA in Biodegradable Organosilica Encapsulates

A core-shell strategy was developed to protect synthetic DNA in organosilica particles encompassing dithiol linkages allowing for a DNA loading of 1.1 wt %. DNA stability tests involving bleach as an oxidant showed that following the procedure DNA was sandwiched between core particles of ca. 450 nm size and a protective outer layer, separating the DNA from the environment. Rapid aging tests at 60 °C and 50% relative humidity revealed that the DNA protected within this material was significantly more stable than nonprotected DNA, with an expected ambient temperature half-life of over 60 years. Still, and due to the presence of the dithiol linkages in the backbone of the organosilica material, the particles degraded in the presence of reducing agents (TCEP and glutathione) and disintegrated within several days in a simulated compost environment, which was employed to test the biodegradability of the material. This is in contrast to DNA encapsulated following state of the art procedures in pure SiO₂ particles, which do not biodegrade in the investigated timeframes and conditions. The results show that synthetic DNA protected within dithiol comprising organosilica particles presents a strategy to store digital data at a high storage capacity for long time frames in a fully biodegradable format.

Miniaturized Raman Instruments for SERS-Based Point-of-Care Testing on Respiratory Viruses

As surface-enhanced Raman scattering (SERS) has been used to diagnose several respiratory viruses (e.g., influenza A virus subtypes such as H1N1 and the new coronavirus SARS-CoV-2), SERS is gaining popularity as a method for diagnosing viruses at the point-of-care. Although the prior and quick diagnosis of respiratory viruses is critical in the outbreak of infectious disease, ELISA, PCR, and RT-PCR have been used to detect respiratory viruses for pandemic control that are limited for point-of-care testing. SERS provides quantitative data with high specificity and sensitivity in a real-time, label-free, and multiplex manner recognizing molecular fingerprints. Recently, the design of Raman spectroscopy system was simplified from a complicated design to a small and easily accessible form that enables point-of-care testing. We review the optical design (e.g., laser wavelength/power and detectors) of commercialized and customized handheld Raman instruments. As respiratory viruses have prominent risk on the pandemic, we review the applications of handheld Raman devices for detecting respiratory viruses. By instrumentation and commercialization advancements, the advent of the portable SERS device creates a fast, accurate, practical, and cost-effective analytical method for virus detection, and would continue to attract more attention in point-of-care testing.

Ferrocene-appended anthraquinone and coumarin as redox-active cytotoxins

Appending of ferrocene (Fc) to biologically-active organic backbones can generate novel multi-functional species for targeting bacteria and cancer. In this work Fc was linked to coumarin and anthraquinone with the goal of harnessing the redox-active Fc centre to generate new compounds that exhibit cytoxicity through the generation of toxic reactive oxygen species (ROS). A Cu(I)-catalyzed azide-alkyne cycloaddition "click" reaction was used to connect the organic and Fc components via a triazole linker. Cyclic voltammetry shows that the Fc potentials are suitable for oxidation by biological hydrogen peroxide to give reactive ferrocenium (Fc⁺) species, which can then generate hydroxyl radicals. The ability of the compounds to generate hydroxyl radicals in the presence of hydrogen peroxide was shown directly using EPR spin-trapping experiments. Furthermore, in vitro studies in MCF-7 breast cancer cells show significant increases in ROS following incubation with the Fc-functionalized compounds. Screening for antibacterial activity produced negative results for all of the Fc compounds, consitent with low levels of hydrogen peroxide typically found in bacteria. By contrast, Fc-coumarin showed cytotoxicity against A549 lung cancer and SKOV3 ovarian cancer cell lines, whereas the parent compound was inactive. This is consistent both with the cytoxic potential of the Fc group and the elevated hydrogen peroxide levels found in many cancers. Interestingly, the anthraquinone compounds showed the opposite effect with the parent compounds showing modest activity against A549 cells, but the Fc compounds being inactive. This demonstrates other potential negative impacts of including Fc, such as significantly increased lipophilicity.

Surface-enhanced Raman scattering: An emerging tool for sensing cellular function

Continuous long-term intracellular imaging and multiplexed monitoring of biomolecular changes associated with key cellular processes remains a challenge for the scientific community. Recently, surface-enhanced Raman scattering (SERS) has been demonstrated as a powerful spectroscopic tool in the field of biology owing to its significant advantages. Some of these include the ability to provide molecule-specific information with exquisite sensitivity, working with small volumes of precious samples, real-time monitoring, and optimal optical contrast. More importantly, the availability of a large number of novel Raman reporters with narrower full width at half maximum (FWHM) of spectral peaks/vibrational modes than conventional fluorophores has created a versatile palette of SERS-based probes that allow targeted multiplex sensing surpassing the detection sensitivity of even fluorescent probes. Due to its nondestructive nature, its applicability has been recognized for biological sensing, molecular imaging, and dynamic monitoring of complex intracellular processes. We critically discuss recent developments in this area with a focus on different applications where SERS has been used for obtaining information that remains elusive for conventional imaging methods. Current reports indicate that SERS has made significant inroads in the field of biology and has the potential to be used for in vivo human applications. This article is categorized under: Diagnostic Tools > In Vitro Nanoparticle-Based Sensing Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Diagnostic Tools > Biosensing Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Quantification of doping state of redox sensitive nanoparticles for probing the invasiveness of cancer cells using surface enhanced Raman scattering

Redox activity is known to regulate migration, invasion, metastasis, proliferation, and vascularization of cancer. Because cancer is heterogeneous, the role of redox activity in different cancers and cancer-related processes vary widely. In this study, water soluble, Tween 80-coated polyaniline (TPAni) nanoparticles were synthesized and used as nano-agents for sensing the redox activities of various cancer cells. To identify the relationship between the redox activity and the aggressiveness of cancer cells, two different cancer cell lines, derived from the same tissue but different with regards to aggressiveness, were selected for study. First, the cancer cell lines were incubated with TPAni nanoparticles, and an absorbance ratio obtained from the cell culture media was used as a colorimetric indicator of the redox activities of the cells. Simultaneously, hydrophobically modified filter papers coated with silver nanosnowflakes (SNSF) were used as sensing substrates for surface enhanced Raman scattering (SERS). SERS spectra obtained from varying concentrations of rhodamine 6G were used to confirm the detection limit of the SNSF-based SERS substrate. Cell culture media containing TPAni nanoparticles were treated with the SNSF-containing SERS substrates to examine the redox activities of the various cancer cell lines.The redox activities of cancer cell lines were confirmed by absorbance spectral analysis, and these redox activities were better identified via an SERS analysis method. A SNSF-containing SERS substrate, fabricated from SNSF and filter paper, was used to sense redox activity in cancer cell lines and to further identify correlations between redox activity and cancer cell line aggressiveness, as indicated by the use of EpCAM as a biomarker. Finally, potential of ​in vivo ​redox activity sensing was also confirmed.

From Raman to SESORRS: moving deeper into cancer detection and treatment monitoring

Raman spectroscopy is a non-invasive technique that allows specific chemical information to be obtained from various types of sample. The detailed molecular information that is present in Raman spectra permits monitoring of biochemical changes that occur in diseases, such as cancer, and can be used for the early detection and diagnosis of the disease, for monitoring treatment, and to distinguish between cancerous and non-cancerous biological samples. Several techniques have been developed to enhance the capabilities of Raman spectroscopy by improving detection sensitivity, reducing imaging times and increasing the potential applicability for in vivo analysis. The different Raman techniques each have their own advantages that can accommodate the alternative detection formats, allowing the techniques to be applied in several ways for the detection and diagnosis of cancer. This feature article discusses the various forms of Raman spectroscopy, how they have been applied for cancer detection, and the adaptation of the techniques towards their use for in vivo cancer detection and in clinical diagnostics. Despite the advances in Raman spectroscopy, the clinical application of the technique is still limited and certain challenges must be overcome to enable clinical translation. We provide an outlook on the future of the techniques in this area and what we believe is required to allow the potential of Raman spectroscopy to be achieved for clinical cancer diagnostics.

Comparison of SERS pH probe responses after microencapsulation within hydrogel matrices

SIGNIFICANCE

Personalized medicine requires the tracking of an individual's metabolite levels over time to detect anomalies and evaluate the body's response to medications. Implanted sensors offer effective means to continuously monitor specific metabolite levels, provided they are accurate, stable over long time periods, and do no harm.

AIM

Four types of hydrogel embedded with pH-sensitive sensors were evaluated for their accuracy, sensitivity, reversibility, longevity, dynamic response, and consistency in static versus dynamic conditions and long-term storage.

APPROACH

Raman spectroscopy was first used to calibrate the intensity of pH-sensitive peaks of the Raman-active hydrogel sensors in a static pH environment. The dynamic response was then assessed for hydrogels exposed to changing pH conditions within a flow cell. Finally, the static pH response after 5 months of storage was determined.

RESULTS

All four types of hydrogels allowed the surface-enhanced Raman spectroscopy (SERS) sensors to respond to the pH level of the local environment without introducing interfering signals, resulting in consistent calibration curves. When the pH level changed, the probes in the gels were slow to reach steady-state, requiring several hours, and response times were found to vary among hydrogels. Only one type, poly(2-hydroxyethyl methacrylate) (pHEMA), lasted five months without significant degradation of dynamic range.

CONCLUSIONS

While all hydrogels appear to be viable candidates as biocompatible hosts for the SERS sensing chemistry, pHEMA was found to be most functionally stable over the long interval tested. Poly(ethylene glycol) hydrogels exhibit the most rapid response to changing pH. Since these two gel types are covalently cross-linked and do not generally degrade, they both offer advantages over sodium alginate for use as implants.

Copper(II) Pyridyl Aminophenolates: Hypoxia-Selective, Nucleus-Targeting Cytotoxins, and Magnetic Resonance Probes

Targeting the low-oxygen (hypoxic) environments found in many tumours by using redox-active metal complexes is a strategy that can enhance efficacy and reduce the side effects of chemotherapies. We have developed a series of Cu^II complexes with tridentate pyridine aminophenolate-based ligands for preferential activation in the reduction window provided by hypoxic tissues. Furthermore, ligand functionalization with a pendant CF₃ group provides a ¹⁹ F spectroscopic handle for magnetic-resonance studies of redox processes at the metal centre and behaviour in cellular environments. The phenol group in the ligand backbone was substituted at the para position with H, Cl, and NO₂ to modulate the reduction potential of the Cu^II centre, giving a range of values below the window expected for hypoxic tissues. The NO₂ -substituted complex, which has the highest reduction potential, showed enhanced cytotoxic selectivity towards HeLa cells grown under hypoxic conditions. Cell death occurs by apoptosis, as determined by analysis of the cell morphology. A combination of ¹⁹ F NMR and ICP-OES indicates localization of the NO₂ complex in HeLa cell nuclei and increased cellular accumulation under hypoxia. This correlates with DNA nuclease activity being the likely origin of cytotoxic activity, as demonstrated by cleavage of DNA plasmids in the presence of the Cu^II nitro complex and a reducing agent. Selective detection of the paramagnetic Cu^II complexes and their diamagnetic ligands by ¹⁹ F MRI suggests hypoxia-targeting theranostic applications by redox activation.

Intracellular pH - Advantages and pitfalls of surface-enhanced Raman scattering and fluorescence microscopy - A review

The value of pH in various parts of protoplasm can affect nearly all aspects of cell functions. Therefore, the determination of intracellular acid-base features is required in many areas of biological and biochemical studies. Because of a significant scientific importance of in vivo intracellular pH measurements, various groups carried out such experiments. In this review article we describe intracellular pH measurements using two the most sensitive optical spectroscopies: surface-enhanced Raman scattering (SERS) and fluorescence. It is reasonable to present these two techniques in one review article because the experimental approach in Raman and fluorescence experiments is relatively similar. The basic theoretical background explaining the mechanism of operation of fluorescence and SERS sensors are discussed and the motivations to carry out intracellular pH measurements are briefly described. Future perspectives in this field are also discussed.

Investigation of the pathway dependent endocytosis of gold nanoparticles by surface-enhanced Raman scattering

Endocytosis is a critical mechanism providing not only internalization of biomacromolecular structures but also communication with the environment where cells reside. Due to being the first step at the interaction interface, the route of cellular uptake has a major role governing the intracellular destinations and behaviors of molecular and non-molecular species including nanoparticles. To this end, various methods employing variety of techniques are investigated. In this study, surface-enhanced Raman spectroscopy (SERS) based approach for the investigation of endocytosis of gold nanoparticles (AuNPs) is reported. Internalization pathways of AuNPs were examined by flow cytometry via specific inhibitors for each endocytosis pathway type using three model cell lines Beas-2b, A549 and PNT1A. Macropinocytosis was blocked by cytochalasin D (CytoD), clathrin mediated endocytosis (CME) by sucrose (Scr), and caveolae mediated endocytosis (CE) by filipin (Fil). The results showed that cell type dependent AuNPs internalization affects not only the response of the cells to the inhibitors but also the obtained SERS spectra. SERS spectra of PNT1A cells treated with inhibitors was influenced most. The inhibition of each endocytosis pathway significantly affected the SERS spectral pattern and the spectral changes in different endocytosis pathways were clearly discriminated from each other. This means that SERS can significantly contribute to the investigation of different endosomal pathways from single living cells without any disruption of the cells or labeling.

Using High-Temporal-Resolution Ambient Data to Investigate Gas-Particle Partitioning of Ammonium over Different Seasons

Ammonium is one of the dominant inorganic water-soluble ions in fine particulate matter (PM_2.5). In this study, source apportionment and thermodynamic equilibrium models were used to analyze the relationship between pH and the partitioning of ammonium (ε(NH₄⁺)) using hourly ambient samples collected from Tianjin, China. We found a "Reversed-S curve" between pH and ε(NH₄⁺) from the ambient hourly aerosol dataset when the theoretical ε(NO₃^-)^* (an index identified in this work) was within specific ranges. A Boltzmann function was then used to fit the Reversed-S curve. For the summer data set, when ε(NO₃^-)^* was between 0.7 and 0.8, the fitted R² was 0.88. Through thermodynamic analysis, we found that the values of k[H⁺]² (k = 3.08 × 10⁴ L² mol^-2) and ε(NO₃^-)^* can influence the pH-ε(NH₄⁺) curve. Under certain situations, the values of k[H⁺]² and ε(NO₃^-)^* are similar to each other, and ε(NH₄⁺) is sensitive to pH, suggesting that ε(NO₃^-)^* plays an important role in affecting the ε(NH₄⁺). During summer, winter, and spring seasons, when the relative humidity was greater than 0.36 and ε(NO₃^-)^* was between 0.8 and 0.95, there was an obvious Reversed-S curve, with R² = 0.60. The theoretical k[H⁺]² and ε(NO₃^-)^* developed in this work can be used to analyze the gas-particle partitioning of ammonia-ammonium and nitrate-nitric acid in the ambient atmosphere. Also, it is the first time that we created the joint source-NH₃/HNO₃ maps to integrate sources, aerosol pH and liquid water content, and ions (altogether in one map), which can provide useful information for designing effective strategies to control particulate matter pollution.

A new class of ratiometric small molecule intracellular pH sensors for Raman microscopy

Intracellular pH (pH_i) homeostasis is intertwined with a myriad of normal cellular behaviors as well as pathological processes. As such, small molecule probes for the measurement of pH_i are invaluable tools for chemical biology, facilitating the study of the role of pH in cellular function and disease. The field of small molecule pH_i sensors has traditionally been dominated with probes based on fluorescent scaffolds. In this study, a series of low molecular weight (<260) oligoyne compounds have been developed which exhibit pH sensitive alkyne stretching frequencies (ν_alkyne) in Raman spectroscopy. The modular design of the compounds enabled tuneability of their pK_a(H) through simple structural modification, such that continuous pH sensitivity is achieved over the range 2-10. Alkyne stretching bands reside in the 'cell-silent' region of the Raman spectrum (1800-2600 cm^-1) and are readily detectable in a cellular environment with subcellular spatial resolution. This enabled the application of a pH sensitive oligoyne compound to the ratiometric sensing of pH_i in prostate cancer (PC3) cells in response to drug treatment. We propose that probes based on Alkyne Tag Raman Imaging offer an entirely new platform for the sensing of pH_i, complementary to fluorescence microscopy.

Effect of the functionalisation agent on the surface-enhanced Raman scattering (SERS) spectrum: Case study of pyridine derivatives

Nowadays, the use of functionalised surface-enhanced Raman scattering (SERS) substrates has become common. These surface modifying agents notably act as Raman reporters, as sensors of biological processes (pH, redox probes) or to increase the sensitivity and/or the specificity of SERS detections. However, the effects of the functionalisation agents are deeply examined in very few studies, even though they can affect the aggregation behaviour of the SERS substrate. Moreover, depending on their concentration and on the pH, their spectral signature can be modified and they can even degrade if stored inappropriately. In this context, this paper aims at emphasising the importance of the different aspects previously listed in the selection of a functionalisation agent. Pyridine derivatives were picked out to highlight these parameters, as some of these compounds are commonly used to be grafted onto SERS substrates. Two widespread syntheses of nanoparticles were selected as SERS substrates: citrate-reduced gold and silver nanoparticles. The surface of the nanoparticles was functionalised with several pyridine derivatives at different concentrations and in several solvents. It was observed that the molecules under study had a concentration-dependent effect on nanoparticle aggregation. A stability study was furthermore conducted in order to determine the best preservation conditions of the grafting solutions. In conclusion, this paper shines a light on the relevance of the investigation of the too-often neglected behaviour of the surface modifying agents. Before their application in SERS analyses, parameters such as the label concentration should therefore be included in an experimental design to optimise the sample preparation.

Investigation of cellular uptake mechanism of functionalised gold nanoparticles into breast cancer using SERS

Gold nanoparticles (AuNPs) are widely used in various applications such as cancer imaging and drug delivery. The functionalisation of AuNPs has been shown to affect their cellular internalisation, accumulation and targeting efficiency. The mechanism of cellular uptake of functionalised AuNPs by different cancer cells is not well understood. Therefore, a detailed understanding of the molecular processes is necessary to improve AuNPs for their selective uptake and fate in specific cellular systems. This knowledge can greatly help in designing nanotags with higher cellular uptake for more selective and specific targeting capabilities with less off-target effects. Here, we demonstrate for the first time a straightforward and non-destructive 3D surface enhanced Raman spectroscopy (SERS) imaging approach to track the cellular uptake and localisation of AuNPs functionalised with an anti-ERα (estrogen receptor alpha) antibody in MCF-7 ERα-positive human breast cancer cells under different conditions including temperature and dynamin inhibition. 3D SERS enabled information rich monitoring of the intracellular internalisation of the SERS nanotags. It was found that ERα-AuNPs were internalised by MCF-7 cells in a temperature-dependent manner suggesting an active endocytosis-dependent mechanism. 3D SERS cell mapping also indicated that the nanotags entered MCF-7 cells using dynamin dependent endocytosis, since dynamin inhibition resulted in the SERS signal being obtained from, or close to, the cell surface rather than inside the cells. Finally, ERα-AuNPs were found to enter MCF-7 cells using an ERα receptor-mediated endocytosis process. This study addresses the role of functionalisation of SERS nanotags in biological environments and highlights the benefits of using 3D SERS for the investigation of cellular uptake processes.

Human cystic fibrosis monocyte derived macrophages display no defect in acidification of phagolysosomes when measured by optical nanosensors

BACKGROUND

Defective macrophage phagolysosomal acidification is implicated in numerous lung diseases including Cystic Fibrosis (CF) and may contribute to defective pathogen killing. Conflicting reports relating to phagolysosomal pH in CF macrophages have been published, in part related to the use of pH-sensitive fluorescent probes where potential inadequacies in experimental design can be a contributing factor (e.g. employing probes with incorrect pKa for the cellular compartment of interest). We developed a reliable method to quantify macrophage phagolysosomal pH using surface-enhanced Raman spectroscopy-based nanosensors.

METHODS

Monocyte-derived macrophages from CF and healthy control participants were incubated with nanosensors. Live cell imaging identified phagocytosed nanosensors, and surface-enhanced Raman spectroscopy was performed using para-mercaptobenzoic acid functionalised gold nanoparticles which produce Raman spectra that change predictably with their environmental pH. Conventional fluorescence spectroscopy was carried out in comparison. Nanosensor localisation to phagolysosomes was confirmed by transmission electron microscopy.

RESULTS

Nanosensors were actively phagocytosed by macrophages into phagolysosomes and acidification occurred rapidly and remained stable for at least 60 min. There was no difference in phagolysosomal pH between healthy control and CF macrophages (5.41 ± 0.11 vs. 5.41 ± 0.20, p > .9999), further confirmed by inhibiting Cystic Fibrosis Transmembrane Conductance Regulator in healthy control monocyte-derived macrophages.

CONCLUSIONS

Optical nanosensors accurately measure macrophage phagolysosomal pH and demonstrate no phagolysosomal acidification defect in human CF monocyte-derived macrophages. Further studies using alveolar macrophages could extend the impact of our findings. Nanosensors represent a novel and precise means to measure organelle functions with widespread potential for the study and monitoring of several lung diseases.

Direct measurement of aerosol pH in individual malonic acid and citric acid droplets under different relative humidity conditions via Raman spectroscopy

Acidity of aerosol particles plays important roles in atmospheric chemistry, in turn, impacting climate system and public health. Current knowledge of acidity in atmosphere aerosols remains fairly scarce largely because of difficulty in direct measurement. On the other hand, indirect methods for estimating aerosol pH are often inconsistent with pH values predicted by thermodynamic models. Recently, a direct Raman spectroscopy method has been reported to determine pH values of acid-conjugate base equilibria systems based on Raman intensity of distinct characteristic peaks of conjugate acid-base pair. Nevertheless, for pure carboxylic acid aerosol particles, such as malonic acid (MA), characteristic peak of its conjugate base cannot be clearly observed in Raman spectra owing to small K_a value (weak acid dissociation constant), which leads to little dissociation of weak acid and low concentration of its conjugated base. As a result, pH of carboxylic acid particles cannot be directly determined by calibrating concentrations of acid and its conjugated base using the above-mentioned method. To address such an issue, we demonstrate a new approach for determining pH values of malonic acid (MA) and citric acid (CA) droplets under different relative humidity (RH) based on calibration curves. We measure Raman intensity ratios of acid solutions at different concentrations and their pH values to establish a calibration curve, and then using the intensity ratio of MA and CA droplets under different RH to determine aerosol particle pH based on calibration curves. Results have shown that aerosol pH of MA droplet decreases with a decreasing RH and pH values ranges from 1.03 to -0.12, when RH value is reduced from 90% to 26%, in good agreement with model prediction values. In addition, we also, for the first time, report pH values of CA droplets under different RH conditions and its pH values range from 1.13 to -0.74 when RH is reduced from 91% to 28%.

Bromide ion-functionalized nanoprobes for sensitive and reliable pH measurement by surface-enhanced Raman spectroscopy

4-Mercaptopyridine (4-Mpy) is a pH reporter molecule commonly used to functionalize nanoprobes for surface-enhanced Raman spectroscopy (SERS) based pH measurements. However, nanoprobes functionalized by 4-Mpy alone have low pH sensitivity and are subject to interference by halide ions in sample media. To improve nanoprobe pH sensitivity and reliability, we functionalized gold nanoparticles (AuNPs) with both 4-Mpy and bromide ion (Br^-). Br^- electrostatically stabilizes protonated 4-Mpy, thus enabling sensitive SERS detection of the protonation state of 4-Mpy as a function of pH while also reducing variability caused by external halide ions. Through optimization of the functionalization parameters, including suspension pH, [4-Mpy], and [Br^-], the developed nanoprobes enable monitoring of pH from 2.1 to 10 with high SERS activity and minimal interference from halide ions within the sample matrix. As a proof of concept, we were able to track nanoprobe location and image the pH distribution inside individual cancer cells. This study provides a novel way to engineer reliable 4-Mpy-functionalized SERS nanoprobes for the sensitive analysis of spatially localized pH features in halide ion-containing microenvironments.

Intracellular redox potential is correlated with miRNA expression in MCF7 cells under hypoxic conditions

Hypoxia is a ubiquitous feature of cancers, encouraging glycolytic metabolism, proliferation, and resistance to therapy. Nonetheless, hypoxia is a poorly defined term with confounding features described in the literature. Redox biology provides an important link between the external cellular microenvironment and the cell's response to changing oxygen pressures. In this paper, we demonstrate a correlation between intracellular redox potential (measured using optical nanosensors) and the concentrations of microRNAs (miRNAs) involved in the cell's response to changes in oxygen pressure. The correlations were established using surprisal analysis (an approach derived from thermodynamics and information theory). We found that measured redox potential changes reflect changes in the free energy computed by surprisal analysis of miRNAs. Furthermore, surprisal analysis identified groups of miRNAs, functionally related to changes in proliferation and metastatic potential that played the most significant role in the cell's response to changing oxygen pressure.

Dynamic imaging of cellular pH and redox homeostasis with a genetically encoded dual-functional biosensor, pHaROS, in yeast

Intracellular pH and redox states are critical for multiple processes and partly determine cell behavior. Here, we developed a genetically encoded dual-function probe, named p H and redox-sensitive fluorescent protein (pHaROS), for simultaneous real-time detection of changes in redox potential and pH in living cells. pHaROS consists of the Arabidopsis flavin mononucleotide-binding fluorescent protein iLOV and an mKATE variant, mBeRFP. Using pHaROS in Saccharomyces cerevisiae cells, we confirmed that H₂O₂ raises the overall redox potential of the cell and found that this increase is accompanied by a decrease in cytosolic pH. Furthermore, we observed spatiotemporal pH and redox homeostasis within the nucleus at various stages of the cell cycle in budding yeast (Saccharomyces cerevisiae) during cellular development and responses to oxidative stress. Importantly, we could tailor pHaROS to specific applications, including measurements in different organelles and cell types and the GSH/GSSG ratio, highlighting pHaROS's high flexibility and versatility. In summary, we have developed pHaROS as a dual-function probe that can be used for simultaneously measuring cellular pH and redox potential, representing a very promising tool for determining the cross-talk between intracellular redox- and pH-signaling processes in yeast and mammalian U87 cell.

Subsurface Chemically Specific Measurement of pH Levels in Biological Tissues Using Combined Surface-Enhanced and Deep Raman

There is much interest in using nanosensors to monitor biologically relevant species such as glucose, or cellular pH, as these often become dysregulated in diseases such as cancer. This information is often inaccessible at depth in biological tissue, due to the highly scattering nature of tissue. Here we show that gold nanoparticles labeled with pH-sensitive reporter molecules can monitor pH at depth in biological tissues. This was achieved using deep Raman spectroscopy (spatially offset Raman and transmission Raman) in combination with surface-enhanced Raman spectroscopy, allowing chemical information to be retrieved significantly deeper than conventional Raman spectroscopy permits. Combining these approaches with chemometrics enabled pH changes to be monitored with an error of ±∼0.1 pH units noninvasively through 22 mm of soft tissue. This development opens the opportunity for the next generation of light-based medical diagnostic methods, such as monitoring of cancers, known to significantly alter pH levels.

Intracellular and Cellular Detection by SERS-Active Plasmonic Nanostructures

Surface-enhanced Raman scattering (SERS), with greatly amplified fingerprint spectra, holds great promise in biochemical and biomedical research. In particular, the possibility of exciting a library of SERS probes and differentially detecting them simultaneously has stimulated widespread interest in multiplexed biodetection. Herein, recent progress in developing SERS-active plasmonic nanostructures for cellular and intracellular detection is summarized. The development of nanosensors with tailored plasmonic and multifunctional properties for profiling molecular and pathological processes is highlighted. Future challenges towards the routine use of SERS technology in quantitative bioanalysis and clinical diagnostics are further discussed.

Redox Is a Global Biodevice Information Processing Modality

Biology is well-known for its ability to communicate through (i) molecularly-specific signaling modalities and (ii) a globally-acting electrical modality associated with ion flow across biological membranes. Emerging research suggests that biology uses a third type of communication modality associated with a flow of electrons through reduction/oxidation (redox) reactions. This redox signaling modality appears to act globally and has features of both molecular and electrical modalities: since free electrons do not exist in aqueous solution, the electrons must flow through molecular intermediates that can be switched between two states - with electrons (reduced) or without electrons (oxidized). Importantly, this global redox modality is easily accessible through its electrical features using convenient electrochemical instrumentation. In this review, we explain this redox modality, describe our electrochemical measurements, and provide four examples demonstrating that redox enables communication between biology and electronics. The first two examples illustrate how redox probing can acquire biologically relevant information. The last two examples illustrate how redox inputs can transduce biologically-relevant transitions for patterning and the induction of a synbio transceiver for two-hop molecular communication. In summary, we believe redox provides a unique ability to bridge bio-device communication because simple electrochemical methods enable global access to biologically meaningful information. Further, we envision that redox may facilitate the application of information theory to the biological sciences.

Exploring the Potentiality of a SERS-Active pH Nano-Biosensor

The merging of the molecular specificity of Raman spectroscopy with the extraordinary optical properties of metallic nanoarchitectures is at the heart of Surface Enhanced Raman Spectroscopy (SERS), which in the last few decades proved its worth as powerful analytical tool with detection limits pushed to the single molecule recognition. Within this frame, SERS-based nanosensors for localized pH measurements have been developed and employed for a wide range of applications. Nevertheless, to improve the performances of such nanosensors, many key issues concerning their assembling, calibration and stability, that could significantly impact on the outcome of the pH measurements, need to be clarified. Here, we report on the detailed characterization of a case study SERS-active pH nanosensor, based on the conjugation of gold nanoparticles with the pH-sensitive molecular probe 4-mercaptobenzoic acid (4MBA). We analyzed and optimized all the aspects of the synthesis procedure and of the operating conditions to preserve the sensor stability and provide the highest responsiveness to pH. Exploiting the dependence of the SERS spectrum on the protonation degree of the carboxylic group at the edge of the 4MBA molecules, we derived a calibration curve for the nanosensor. The extrapolated working point, i.e., the pH value corresponding to the highest sensitivity, falls at pH 5.6, which corresponds to the pKa value of the molecule confined at the nanoparticle surface. A shift of the pKa of 4MBA, observed on the molecules confined at the nanostructured interface respect to the bulk counterpart, unveils the opportunity to assembly a SERS-based pH nanosensor with the ability to select its working point in the sensitivity region of interest, by acting on the nanostructured surface on which the molecular probe is confined. As a proof-of-concept, the nanosensor was successfully employed to measure the extracellular pH of normal and cancer cells, demonstrating the capability to discriminate between them.

Polypyrrole⁻Methyl Orange Raman pH Sensor

An easy-to-prepare pH sensor based on electrochemically obtained polypyrrole doped with methyl orange ions is described. It enables the determination of a pH value in the 3-13 range for volumes below 1 µL. In a wide pH range, resonance and pre-resonance methyl orange Raman spectra, excited with the 514.5 nm line of an Ar+ laser, changed noticeably in function of H+ concentration. Two types of measurements were performed. In the first case, Raman spectra of the analyzed solutions were collected for samples placed on the sensor surface using a confocal microscope equipped with a 10x objective. Next, measurements were conducted for the same samples without the sensor. On the basis of these spectra, partial least-squares models were elaborated and validated. Relative standard errors of prediction for calibration, validation, and test samples were found to be in the 3.7%-3.9% range. An analogous model build using spectra registered without the sensor was characterized by slightly worse parameters.

High-Resolution Data Sets Unravel the Effects of Sources and Meteorological Conditions on Nitrate and Its Gas-Particle Partitioning

Nitrate is one of the most abundant inorganic water-soluble ions in fine particulate matter (PM_2.5). However, the formation mechanism of nitrate in the ambient atmosphere, especially the impacts of its semivolatility and the various existing forms of nitrogen, remain under-investigated. In this study, hourly ambient observations of speciated PM_2.5 components (NO₃^-, SO₄^2-, etc.) were collected in Tianjin, China. Source contributions were analyzed by PMF/ME2 (Positive Matrix Factorization using the Multilinear Engine 2) program, and pH were estimated by ISORROPIA-II, to investigate the relationship between pH and nitrate. Five sources (factors) were resolved: secondary sulfate (SS), secondary nitrate (SN), dust, vehicle and coal combustion. SN and pH showed a triangle-shaped relationship. When SS was high, the fraction of nitrate partitioning into the aerosol phase exhibits a characteristic "S-curve" relationship with pH for different seasons. An index ( I_TL) is developed and combined with pH to explore the sensitive regions of "S-curve". Controlling the emissions of anions (SO₄^2-, Cl^-), cations (Ca²⁺, Mg²⁺, etc.) and gases (NO _x, NH₃, SO₂, etc.) will change pH, potentially reducing or increasing SN. The findings of this work provide an effective approach for exploring the formation mechanisms of nitrate under different influencing factors (sources, pH, and I_RL).

Quantitative Evaluation of Surface-Enhanced Raman Scattering Nanoparticles for Intracellular pH Sensing at a Single Particle Level

Intracellular pH is one of the key factors for understanding various biological processes in biological cells. Plasmonic gold and silver nanoparticles (NPs) have been extensively studied for surface-enhanced Raman scattering (SERS) applications for pH sensing as a local pH probe in a living cell. However, the SERS performance of NPs depends on material, size, and shape, which can be controlled by chemical synthesis. Here, we synthesized 18 types of gold and silver NPs with different morphologies such as sphere, rod, flower, star, core/shell, hollow, octahedra, core/satellites, and chainlike aggregates, and quantitatively compared their SERS performance for pH sensing. The SERS intensity from the most commonly utilized SERS probe molecule ( para-mercaptobenzoic acid, p-MBA) for pH sensing was measured at the single nanoparticle level under the same measurement parameters such as low laser power (0.5 mW/μm²), short integration time (100 ms) at wavelengths of 405, 488, 532, 584, 676, and 785 nm. In our measurement, the Ag chain, Ag core/satellites, Ag@Au core/satellites, and Au core/satellites nanoassemblies showed efficient pH sensing at the single particle level. By using p-MBA-conjugated Au@Ag core/satellites, we performed time-lapse pH measurements during apoptosis of HeLa cells. These experimental results confirmed that the pH measurement using p-MBA-conjugated Au@Ag core/satellites can be applied for long-term measurements of intracellular pH during cellular events.

Nanoparticles and intracellular applications of surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectrocopy (SERS) offers ultrasensitive vibrational fingerprinting at the nanoscale. Its non-destructive nature affords an ideal tool for interrogation of the intracellular environment, detecting the localisation of biomolecules, delivery and monitoring of therapeutics and for characterisation of complex cellular processes at the molecular level. Innovations in nanotechnology have produced a wide selection of novel, purpose-built plasmonic nanostructures capable of high SERS enhancement for intracellular probing while microfluidic technologies are being utilised to reproducibly synthesise nanoparticle (NP) probes at large scale and in high throughput. Sophisticated multivariate analysis techniques unlock the wealth of previously unattainable biomolecular information contained within large and multidimensional SERS datasets. Thus, with suitable combination of experimental techniques and analytics, SERS boasts enormous potential for cell based assays and to expand our understanding of the intracellular environment. In this review we trace the pathway to utilisation of nanomaterials for intracellular SERS. Thus we review and assess nanoparticle synthesis methods, their toxicity and cell interactions before presenting significant developments in intracellular SERS methodologies and how identified challenges can be addressed.

Exploring the margins of SERS in practical domain: An emerging diagnostic modality for modern biomedical applications

Excellent multiplexing capability, molecular specificity, high sensitivity and the potential of resolving complex molecular level biological compositions augmented the diagnostic modality of surface-enhanced Raman scattering (SERS) in biology and medicine. While maintaining all the merits of classical Raman spectroscopy, SERS provides a more sensitive and selective detection and quantification platform. Non-invasive, chemically specific and spatially resolved analysis facilitates the exploration of SERS-based nano probes in diagnostic and theranostic applications with improved clinical outcomes compared to the currently available so called state-of-art technologies. Adequate knowledge on the mechanism and properties of SERS based nano probes are inevitable in utilizing the full potential of this modality for biomedical applications. The safety and efficiency of metal nanoparticles and Raman reporters have to be critically evaluated for the successful translation of SERS in to clinics. In this context, the present review attempts to give a comprehensive overview about the selected medical, biomedical and allied applications of SERS while highlighting recent and relevant outcomes ranging from simple detection platforms to complicated clinical applications.

Surface-Enhanced Raman Spectroscopy for Bioanalysis: Reliability and Challenges

Surface-enhanced Raman spectroscopy (SERS) inherits the rich chemical fingerprint information on Raman spectroscopy and gains sensitivity by plasmon-enhanced excitation and scattering. In particular, most Raman peaks have a narrow width suitable for multiplex analysis, and the measurements can be conveniently made under ambient and aqueous conditions. These merits make SERS a very promising technique for studying complex biological systems, and SERS has attracted increasing interest in biorelated analysis. However, there are still great challenges that need to be addressed until it can be widely accepted by the biorelated communities, answer interesting biological questions, and solve fatal clinical problems. SERS applications in bioanalysis involve the complex interactions of plasmonic nanomaterials with biological systems and their environments. The reliability becomes the key issue of bioanalytical SERS in order to extract meaningful information from SERS data. This review provides a comprehensive overview of bioanalytical SERS with the main focus on the reliability issue. We first introduce the mechanism of SERS to guide the design of reliable SERS experiments with high detection sensitivity. We then introduce the current understanding of the interaction of nanomaterials with biological systems, mainly living cells, to guide the design of functionalized SERS nanoparticles for target detection. We further introduce the current status of label-free (direct) and labeled (indirect) SERS detections, for systems from biomolecules, to pathogens, to living cells, and we discuss the potential interferences from experimental design, measurement conditions, and data analysis. In the end, we give an outlook of the key challenges in bioanalytical SERS, including reproducibility, sensitivity, and spatial and time resolution.

Au-Protected Ag Core/Satellite Nanoassemblies for Excellent Extra-/Intracellular Surface-Enhanced Raman Scattering Activity

Silver nanoparticles (AgNPs) and their assembled nanostructures such as core/satellite nanoassemblies are quite attractive in plasmonic-based applications. However, one biggest drawback of the AgNPs is the poor chemical stability which also greatly limits their applications. We report fine Au coating on synthesized quasi-spherical silver nanoparticles (AgNSs) with few atomic layers to several nanometers by stoichiometric method. The fine Au coating layer was confirmed by energy-dispersive X-ray spectroscopy elemental mapping and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. The optimized minimal thickness of Au coating layer on different sized AgNSs (22 nm Ag@0.9 nm Au, 44 nm Ag@1.8 nm Au, 75 nm Ag@2.9 nm Au, and 103 nm Ag@0.9 nm Au) was determined by extreme chemical stability tests using H₂O₂, NaSH, and H₂S gas. The thin Au coating layer on AgNSs did not affect their plasmonic-based applications. The core/satellite assemblies based on Ag@Au NPs showed the comparable SERS intensity and uniformity three times higher than that of noncoated Ag core/satellites. The Ag@Au core/satellites also showed high stability in intracellular SERS imaging for at least two days, while the SERS of the noncoated Ag core/satellites decayed significantly. These spherical Ag@Au NPs can be widely used and have great advantages in plasmon-based applications, intracellular SERS probes, and other biological and analytical studies.

Bioanalytical Measurements Enabled by Surface-Enhanced Raman Scattering (SERS) Probes

Since its discovery in 1974, surface-enhanced Raman scattering (SERS) has gained momentum as an important tool in analytical chemistry. SERS is used widely for analysis of biological samples, ranging from in vitro cell culture models, to ex vivo tissue and blood samples, and direct in vivo application. New insights have been gained into biochemistry, with an emphasis on biomolecule detection, from small molecules such as glucose and amino acids to larger biomolecules such as DNA, proteins, and lipids. These measurements have increased our understanding of biological systems, and significantly, they have improved diagnostic capabilities. SERS probes display unique advantages in their detection sensitivity and multiplexing capability. We highlight key considerations that are required when performing bioanalytical SERS measurements, including sample preparation, probe selection, instrumental configuration, and data analysis. Some of the key bioanalytical measurements enabled by SERS probes with application to in vitro, ex vivo, and in vivo biological environments are discussed.

Raman spectroscopy and regenerative medicine: a review

The field of regenerative medicine spans a wide area of the biomedical landscape-from single cell culture in laboratories to human whole-organ transplantation. To ensure that research is transferrable from bench to bedside, it is critical that we are able to assess regenerative processes in cells, tissues, organs and patients at a biochemical level. Regeneration relies on a large number of biological factors, which can be perturbed using conventional bioanalytical techniques. A versatile, non-invasive, non-destructive technique for biochemical analysis would be invaluable for the study of regeneration; and Raman spectroscopy is a potential solution. Raman spectroscopy is an analytical method by which chemical data are obtained through the inelastic scattering of light. Since its discovery in the 1920s, physicists and chemists have used Raman scattering to investigate the chemical composition of a vast range of both liquid and solid materials. However, only in the last two decades has this form of spectroscopy been employed in biomedical research. Particularly relevant to regenerative medicine are recent studies illustrating its ability to characterise and discriminate between healthy and disease states in cells, tissue biopsies and in patients. This review will briefly outline the principles behind Raman spectroscopy and its variants, describe key examples of its applications to biomedicine, and consider areas of regenerative medicine that would benefit from this non-invasive bioanalytical tool.

Monitoring Dynamic Cellular Redox Homeostasis Using Fluorescence-Switchable Graphene Quantum Dots

Monitoring cellular redox homeostasis is critical to the understanding of many physiological functions ranging from immune reactions to metabolism, as well as to the understanding of pathological development ranging from tumorigenesis to aging. Nevertheless, there is currently a lack of appropriate probes for this ambition, which should be reversibly, sensitively, and promptly responsive to a wide range of physiological oxidants and reductants. In this work, a redox-sensitive fluorescence-switchable probe is designed based on graphene quantum dots (GQDs) functionalized with a chelated redox Fe²⁺/Fe³⁺ couple. The underlying mechanism is investigated and discussed. The high sensitivity and fast response are attributable to the fact that the GQD's photoluminescence is highly sensitive to photon-induced electron transfer because of its ultrasmall size and associated prominent quantum confinement effect. Also taking advantages of GQDs' excellent photostability, biocompatibility, and readiness for cell uptake, our reversibly tunable fluorescence probe is employed to monitor in real time the triggered dynamic change of the intracellular redox state. This addition to the limited arsenal of available redox probes shall be useful to the still poorly understood redox biology, as well as for monitoring environment or chemical processes involving redox reactions.

Targeted SERS nanosensors measure physicochemical gradients and free energy changes in live 3D tumor spheroids

Use of multicellular tumor spheroids (MTS) to investigate therapies has gained impetus because they have potential to mimic factors including zonation, hypoxia and drug-resistance. However, analysis remains difficult and often destroys 3D integrity. Here we report an optical technique using targeted nanosensors that allows in situ 3D mapping of redox potential gradients whilst retaining MTS morphology and function. The magnitude of the redox potential gradient can be quantified as a free energy difference (ΔG) and used as a measurement of MTS viability. We found that by delivering different doses of radiotherapy to MTS we could correlate loss of ΔG with increasing therapeutic dose. In addition, we found that resistance to drug therapy was indicated by an increase in ΔG. This robust and reproducible technique allows interrogation of an in vitro tumor-model's bioenergetic response to therapy, indicating its potential as a tool for therapy development.

Highly stable SERS pH nanoprobes produced by co-solvent controlled AuNP aggregation

Production of gold nanoparticle (AuNP) surface-enhanced Raman spectroscopy (SERS) nanoprobes requires replicable aggregation to produce multimers with high signal intensity. Herein, we illustrate a novel, yet simple, approach to produce SERS nanoprobes through control of co-solvent composition. AuNP multimers were produced by mixing AuNP monomers in water : ethanol co-solvent for variable periods of time. By varying the water : ethanol ratio and the amount of 4-mercaptobenzoic acid (4-MBA) present, the aggregation rate can be systematically controlled. Thiolated poly(ethylene glycol) was then added to halt the aggregation process and provide steric stability. This approach was used to produce pH nanoprobes with excellent colloidal stability in high ionic strength environments and in complex samples. The pH probe exhibits broad pH sensitivity over the range 6-11 and we calculate that a single AuNP dimer in a 35 fL volume is sufficient to generate a detectable SERS signal. As a proof-of-concept, the probes were used to detect the intracellular pH of human prostate cancer cells (PC-3). The internalized probes exhibit a strong 4-MBA signal without any interfering bands from either the cells or the culture media and produce exceptionally detailed pH maps. pH maps obtained from 19 xy surface scans and 14 yz depth scans exhibit highly consistent intracellular pH in the range of 5 to 7, thus indicating the greater reliability and reproducibility of our pH probes compared with other probes previously reported in the literature. Our water : ethanol co-solvent production process is fast, simple, and efficient. Adjustment of solvent composition may become a powerful way to produce SERS tags or nanoprobes in the future.

Mixed-monolayer glyconanoparticles for the detection of cholera toxin by surface enhanced Raman spectroscopy

We have produced silver glyconanoparticles for the sensitive (56 ng mL ^-1), low volume and rapid detection of cholera toxin B-subunit (CTB) in synthetic freshwater (simulating the ion compositions of natural waters in which CTB could be found). This is achieved by monitoring the changes in surface enhanced Raman scattering (SERS) intensity of a Raman reporter bound to the glyconanoparticle surface. The particles selectively aggregate upon interaction with CTB, causing an increase in the measured SERS signal. The particles are designed to mimic the interactions involving the cell surface GM1 ganglioside and CTB. This is achieved by using a combination of polyethylene glycol linkers terminated with either galactose or sialic acid.

Measuring the effects of fractionated radiation therapy in a 3D prostate cancer model system using SERS nanosensors

Using Surface Enhanced Raman Spectroscopy to measure cell death caused by radiation in a 3D model of prostate cancer.

Chemical analysis of multicellular tumour spheroids

Conventional two dimensional (2D) monolayer cell culture has been considered the 'gold standard' technique for in vitro cellular experiments. However, the need for a model that better mimics the three dimensional (3D) architecture of tissue in vivo has led to the development of Multicellular Tumour Spheroids (MTS) as a 3D tissue culture model. To some extent MTS mimic the environment of in vivo tumours where, for example, oxygen and nutrient gradients develop, protein expression changes and cells form a spherical structure with regions of proliferation, senescence and necrosis. This review focuses on the development of techniques for chemical analysis of MTS as a tool for understanding in vivo tumours and a platform for more effective drug and therapy discovery. While traditional monolayer techniques can be translated to 3D models, these often fail to provide the desired spatial resolution and z-penetration for live cell imaging. More recently developed techniques for overcoming these problems will be discussed with particular reference to advances in instrument technology for achieving the increased spatial resolution and imaging depth required.