Simultaneous Chemical Mapping of Live Biofilm Microenvironmental pH and Hydrogen Peroxide in Real Time with a Triple Scanning Electrochemical Microscopy Tip

Enzyme-Modified Microelectrode for Simultaneous Local Measurements of O2 and pH

The use of miniaturized probes opens a new dimension in the analysis of (bio)chemical processes, enabling the possibility to perform measurements with local resolution. In addition, multiparametric measurements are highly valuable for a holistic understanding of the investigated process. Therefore, different strategies have been suggested for simultaneous local measurements of various parameters. Electroanalytical methods are a powerful strategy in this direction. However, they have been mainly restricted to coupling concurrent independent measurements with different miniaturized probes. Here, we present an enzymatic microbiosensor for the simultaneous detection of O2 and pH. The sensing strategy is based on the pH-dependent bioelectrocatalytic process associated with O2 reduction at a gold microelectrode modified with a multicopper oxidase. After initial investigations of the bioelectrocatalytic reaction over gold macroelectrodes, the fabrication and characterization of micrometer-sized probes are presented. The microbioelectrode exhibits a linear current increase with O2 concentration extending to 17.2 mg L-1, with a sensitivity of (5.56 ± 0.13) nA L mg-1 and a limit of detection of (0.5 ± 0.3) mg L-1. Moreover, a linear response allowing pH detection is obtained between pH 5.2 and 7.5 with a slope of -(47 ± 8) mV per pH unit. In addition, two proof-of-concept analytical examples are shown, demonstrating the capability of the developed sensing system for simultaneous local measurements of O2 and pH. Compared with other miniaturized probes reported before for simultaneous detection, our strategy stands out as the two investigated parameters are acquired from the very same measurement. This strategy greatly simplifies the analytical setup and for the first time provides truly simultaneous local detection in the micrometer scale.

Electroanalytical Strategies for Local pH Sensing at Solid-Liquid Interfaces and Biointerfaces

Obtaining analytical information about chemical species at interfaces is fundamentally important to improving our understanding of chemical reactions and biological processes. pH at solid-liquid interfaces is found to deviate from the bulk solution value, for example, in electrocatalytic reactions at surfaces or during the corrosion of metals. Also, in the vicinity of living cells, metabolic reactions or cellular responses cause changes in pH at the extracellular interface. In this review, we collect recent progress in the development of sensors with the capability to detect pH at or close to solid-liquid and bio interfaces, with spatial and time resolution. After the two main principles of pH detection are presented, the different classes of molecules and materials that are used as active components in these sensors are described. The review then focuses on the reported electroanalytical techniques for local pH sensing. As application examples, we discuss model studies that exploit local pH sensing in the area of electrocatalysis, corrosion, and cellular interfaces. We conclude with a discussion of key challenges for wider use of this analytical approach, which shows promise to improve the mechanistic understanding of reactions and processes at realistic interfaces.

Triple-function carbon-based Ca2+ ion-selective pH ring microelectrode to study real-time bacteria-mediated hydroxyapatite corrosion

BACKGROUND

The local pH change mediated by the pathogenic bacterial species Streptococcus mutans plays a significant role in the corrosion of hydroxyapatite (HA) present in the tooth in the dynamic oral cavity. The acid produced by the bacteria decreases the local pH and releases Ca2+ ions from the HA. We studied the bacteria-mediated demineralization of HA by scanning electrochemical microscopy (SECM) after growing S. mutans biofilm on HA for 7 days.

RESULTS

We notably developed a triple-function SECM-compatible tip that could be positioned above the biofilm. It can also measure the pH and [Ca2+] change simultaneously above the biofilm-HA substrate. The triple-function SECM tip is a combination of a potentiometric pH sensor deposited with iridium oxide and a dual-function carbon-based Ca2+ ion-selective membrane electrode with a slope of 67 mV/pH and 34.3 mV/log [Ca2+], respectively. The distance-controlled triple-function SECM tip monitored real-time pH and [Ca2+] changes 30 μm above the S. mutans biofilm. The high temporal resolution pH data demonstrated that after approximately 20 min of sucrose addition, S. mutans started to produce acid to titrate the solution buffer, causing a pH change from 7.2 to 6.5 for HA and from 7.2 to 5 for the glass substrate. We observed that, after 30 min of acid production, ∼300 μM of Ca2+ ions were increased at pH 6.5 above the biofilm surface as a result of the pH change in the local microenvironment. After the release of Ca2+ from HA, the pH environment again shifted toward the neutral side, from 6.5 to 7.2. Therefore, precipitation of Ca2+ happens at the top of the biofilm, thus corroding the HA from underneath. For a glass substrate, in contrast, no Ca2+ ions were released, and the pH did not change back to 7.2. We were able to observe the dynamics of the HA demineralization-remineralization process simultaneously with our newly developed triple-function SECM tip or microprobe.

SIGNIFICANCE

This technique could notably advance the study of similar complex processes, such as bacteria-mediated corrosion in biomedical and environmental contexts.

Integration of Scanning Electrochemical Microscopy and Scanning Electrochemical Cell Microscopy in a Bifunctional Nanopipette toward Simultaneous Mapping of Activity and Selectivity in Electrocatalysis

Scanning electrochemical microscopy (SECM) and scanning electrochemical cell microscopy (SECCM) were integrated in a single bifunctional probe for simultaneous mapping of the oxygen reduction current and the oxidation current of the produced H2O2. The dual probe is fabricated from a double-barrel θ capillary, comprising one open barrel filled with the electrolyte and another filled with pyrolytic carbon. Pt is deposited with a gas injection system (GIS) at the end of the carbon barrel. The probe integrates the advantages of both SECM and SECCM by forming an electrochemical droplet cell that embeds the Pt working electrode of the carbon barrel directly into the electrolyte meniscus formed upon sample contact from the electrolyte barrel. The versatility of the dual probe is demonstrated by mapping the oxygen reduction reaction (ORR) current and the H2O2 oxidation current of a Pt microstrip on a gold substrate. This allows simultaneous localized electrochemical measurements, highlighting the potential of the dual probe for broader applications in characterizing the electrocatalytic properties of materials.

Characterization and morphological methods for oral biofilm visualization: where are we nowadays?

The oral microbiome represents an essential component of the oral ecosystem whose symbiotic relationship contributes to health maintenance. The biofilm represents a state of living of microorganisms surrounding themselves with a complex and tridimensional organized polymeric support and defense matrix. The substrates where the oral biofilm adhere can suffer from damages due to the microbial community metabolisms. Therefore, microbial biofilm represents the main etiological factor of the two pathologies of dental interest with the highest incidence, such as carious pathology and periodontal pathology. The study, analysis, and understanding of the characteristics of the biofilm, starting from the macroscopic structure up to the microscopic architecture, appear essential. This review examined the morphological methods used through the years to identify species, adhesion mechanisms that contribute to biofilm formation and stability, and how the action of microbicidal molecules is effective against pathological biofilm. Microscopy is the primary technique for the morphological characterization of biofilm. Light microscopy, which includes the stereomicroscope and confocal laser microscopy (CLSM), allows the visualization of microbial communities in their natural state, providing valuable information on the spatial arrangement of different microorganisms within the biofilm and revealing microbial diversity in the biofilm matrix. The stereomicroscope provides a three-dimensional view of the sample, allowing detailed observation of the structure, thickness, morphology, and distribution of the various species in the biofilm while CLSM provides information on its three-dimensional architecture, microbial composition, and dynamic development. Electron microscopy, scanning (SEM) or transmission (TEM), allows the high-resolution investigation of the architecture of the biofilm, analyzing the bacterial population, the extracellular polymeric matrix (EPS), and the mechanisms of the physical and chemical forces that contribute to the adhesion of the biofilm to the substrates, on a nanometric scale. More advanced microscopic methodologies, such as scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HR-TEM), and correlative microscopy, have enabled the evaluation of antibacterial treatments, due to the potential to reveal the efficacy of different molecules in breaking down the biofilm. In conclusion, evidence based on scientific literature shows that established microscopic methods represent the most common tools used to characterize biofilm and its morphology in oral microbiology. Further protocols and studies on the application of advanced microscopic techniques are needed to obtain precise details on the microbiological and pathological aspects of oral biofilm.

Quantifying picomoles of analyte from less than 100 live bacteria: A novel method with a buffering hydrogel as an electrochemical cell

Microenvironmental changes in the chemical surrounding of bacterial cells might have a profound impact on the ecology of biofilms. However, quantifying total amount of picomoles of analyte from a miniscule number of bacteria is an analytical challenge. Here we provide a novel microliter volume hydrogel based electrochemical cell platform suitable of coulometrically measuring hydrogen peroxide (H2O2) produced by less than 100 cells of Streptococcus sanguinis, a relevant member of the healthy oral microbiome. A morpholine moiety was incorporated into the polymer structure of the hydrogel to create a controlled microenvironment at biological pH. We calculated the buffering capacity of this hydrogel as 0.257 ± 0.135 m o l H N O 3 m o l M E A × Δ p H over the pH range of 7.2-6.2 by using a novel method designed for buffering hydrogels. The H2O2 sensors coated in microliter volume of buffering hydrogel showed no change in sensitivity within the pH range of 7.0-3.0, allowing for H2O2 measurements of S. sanguinis independent of any acid they produce. The novel platform was able to measure down to 22.7 ± 3.5 pmol H2O2 produced by less than 100 bacterial cells, which would otherwise not be attainable in large solution-based assays. These findings indicate that this is a suitable platform for quantifying metabolites from sub-milligram biological samples and may even be suitable for direct analysis of raw biofilms samples with little to no sample pretreatment.