Visual pH Sensors: From a Chemical Perspective to New Bioengineered Materials

Abstract

Many human activities and cellular functions depend upon precise pH values, and pH monitoring is considered a fundamental task. Colorimetric and fluorescence sensors for pH measurements are chemical and biochemical tools able to sense protons and produce a visible signal. These pH sensors are gaining widespread attention as non-destructive tools, visible to the human eye, that are capable of a real-time and in-situ response. Optical “visual” sensors are expanding researchers’ interests in many chemical contexts and are routinely used for biological, environmental, and medical applications. In this review we provide an overview of trending colorimetric, fluorescent, or dual-mode responsive visual pH sensors. These sensors include molecular synthetic organic sensors, metal organic frameworks (MOF), engineered sensing nanomaterials, and bioengineered sensors. We review different typological chemical entities of visual pH sensors, three-dimensional structures, and signaling mechanisms for pH sensing and applications; developed in the past five years. The progression of this review from simple organic molecules to biological macromolecules seeks to benefit beginners and scientists embarking on a project of pH sensing development, who needs background information and a quick update on advances in the field. Lessons learned from these tools will aid pH determination projects and provide new ways of thinking for cell bioimaging or other cutting-edge in vivo applications.

A New ‘Off–On’ System Based on Core‐Substituted Naphthalene Diimide with Dimethylamine for Reversible Acid–Base Sensing

“…The probe 4,9-bis(dimethylamino)-2,7-dioctylbenzo[lmn] [3,8]phenanthroline-1,3,6,8-(2H,7H)-tetraone (DDPT 1) was synthesized with substituted dimethylamine at the core of NDI. The molecule was successfully synthesized and characterized showing the reversible response to trifluoroacetic acid which acts as proton source.…”

“… [1] The cell homeostasis regulation and various metabolic pathways are governed by pH, showing the importance to not only being limited to chemistry but also applying to biochemistry, cellular biology and drug delivery systems. [ 2 , 3 ] To understand pH changes, different glass‐pH‐electrodes are widely used as pH sensors, measuring the pH in a wide range from acidic over neutral to basic media. The glass pH electrodes are free from interference, show low detection limit, and long‐term stability with excellent reproducibility.…”

A Water Soluble 2-Phenyl-5-(pyridin-3-yl)-1,3,4-oxadiazole Based Probe: Antimicrobial Activity and Colorimetric/Fluorescence pH Response

“…Optical probes offer in situ observation, real-time and fast-response monitoring, sensitivity, selectivity [ 1 , 2 , 3 ], and even by the non-invasive approach required for living tissues [ 4 , 5 , 6 ]. Colorimetric/fluorescent tools revolutionized the ability to probe biological dynamics at the cellular level on several platforms: small-molecule dyes, metal complexes, and fluorescent proteins [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ].…”

pH-Responsive N^C-Cyclometalated Iridium(III) Complexes: Synthesis, Photophysical Properties, Computational Results, and Bioimaging Application

“…On the contrary, the use of sensors operating in FLIM/PLIM mode does not need an internal standard for the quantitative measurements because emission lifetime response on external stimuli is independent of the probe concentration. Luminescent pH probes for microscopy are usually designed as fluorescence organic dyes [ 11 , 12 , 13 ] or phosphorescent organometallic coordination compounds [ 14 , 15 ], bearing pH-sensitive groups in their ligand environment. Organic fluorescent sensors appear to have poor photostability, small Stokes shift, low singlet lifetime, and weak lifetime response on pH variations (the best examples are SNARF-derivatives [ 16 ], 2.21 ns (pH 10)/0.49 ns (pH 5.5)).…”