Local pH monitoring of small cluster of cells using a fiber-optic dual-core micro-probe

Abstract

Biological studies of tissues and cells have enabled numerous discoveries, but these studies still bear potential risks of invalidation because of cell heterogeneity. Through high-accuracy techniques, recent studies have demonstrated that discrepancies do exist between the results from low-number-cell studies and cell-population-based results. Thus the urgent need to re-evaluate key principles on limited number of cells has been provoked. In this study, a novel designed dual-core fiber-optic pH micro-probe was fabricated and demonstrated for niche environment pH sensing with high spatial resolution. An organic-modified silicate (OrMoSils) sol-gel thin layer was functionalized by entrapping a pH indicator, 2′, 7′-Bis (2-carbonylethyl)-5(6)-carboxyfluorescein (BCECF), on a ~70 μm sized probe tip. Good linear correlation between fluorescence ratio of I560 nm/I640 nm and intercellular pH values was obtained within a biological-relevant pH range from 6.20 to 7.92 (R2 = 0.9834), and with a pH resolution of 0.035 ± 0.005 pH units. The probe’s horizontal spatial resolution was demonstrated to be less than 2mm. Moreover, the probe was evaluated by measuring the localized extracellular pH changes of cultured human lung cancer cells (A549) when exposed to titanium dioxide nanoparticles (TiO2 NPs). Results showed that the probe has superior capability for fast, local, and continual monitoring of a small cluster of cells, which provides researchers a fast and accurate technique to conduct local pH measurements for cell heterogeneity-related studies.

Characterization of a fast response fiber-optic pH sensor and measurements in a biological application

“…The determination of the pH level is very important across a broad range of applications today: in life sciences, environmental monitoring, biomedical research and widely in industry globally. For example, there is particular interest in detecting pH values using optical techniques for areas as diverse as biological studies of tissues 1 and cells 2,3 , process control in bioreactors [4][5][6][7][8] , seawater analysis 9,10 or even corrosion monitoring 11 . Classical pH glass electrodes have been well known and used for the measurement of pH values over many decades.…”

An Extended Gate Field Effect Transistor (EGFET)-based pH Microsensor Utilizing a Polypyyrole Coated pH Microelectrode

“…Generally, pH microprobes have disadvantages such as limited pH measurement range, long response time for meaningful signal, and sensitivity. H+ ion selective field effect transistors (FET) [4], optical pH sensors [5] and microelectrode pH sensors [6] are more outstanding examples and have been used successfully in certain studies in the literature. Although ion-selective FET (ISFET) based pH sensors are relatively easy to miniaturize, they have disadvantages such as low sensitivity and long signal stabilization time.…”

Unveil early-stage nanocytotoxicity by a label-free single cell pH nanoprobe

“…A series of novel optical microprobes have recently been developed in our group for realtime, label-free pH measurements. [38][39][40][41] Recently, we have developed a sensor with a tip diameter of about 3 μm and a sensing length of approximately 5 μm, which enabled a true intracellular detection capability. 42 Detection sensitivity of ~0.038 pH unit and detection (signal equilibrium) time of ~20 s was successfully achieved.…”

“…Nanoparticles (NPs) induced cytotoxicity has drawn increasing health and environmental concerns, and it is also a challenging topic in single-cell research for largely heterogeneous and unpredictable cellular behavior that may be involved. Destructed cell membranes, [43][44][45] generation of reactive oxygen species (ROS), [46][47][48][49] compromised metabolic activities, 50,51 degraded genetic materials, 52,53 as well as interrupted pH i , 41,54 were frequently observed during the nanocytotoxicity process. Unfortunately, most commercially available assays are based on cell populations, which not only sacrifice the precision and instantaneity of the assay, but also loses most of the heterogeneous information hidden behind the cell-cell differences.…”