Colorimetric pH-sensing of artificial gastric fluid using naphthalimide-based CH acids

Area of Science:

• Organic Chemistry
• Analytical Chemistry
• Materials Science

Background:

• Development of sensitive and selective pH probes is crucial for chemical and biological analyses.
• Naphthalimide derivatives are known for their photophysical properties, making them suitable scaffolds for sensor design.
• Existing pH sensors may lack selectivity or stability in complex matrices like biological fluids.

Purpose of the Study:

• To synthesize and characterize novel colorimetric pH-sensing probes based on naphthalimide malonate derivatives.
• To investigate the pH-sensing characteristics and mechanisms of these novel probes.
• To develop a practical application for pH sensing in artificial gastric fluid.

Main Methods:

• Synthesis of naphthalimide malonate derivatives via reaction of 4-bromo-1,8-naphthalimide with malonates (malononitrile, ethyl cyanoacetate, diethyl malonate).
• Spectroscopic analysis (UV-Vis, fluorescence) and NMR spectroscopy (¹H, ¹³C) to determine pH-sensing behavior and mechanism.
• Determination of pKa values through pH-dependent absorbance curve fitting.
• Integration of the most effective probe into a paper-based test strip for practical application.

Main Results:

• Synthesized three naphthalimide malonate derivatives with distinct pH-sensing properties based on CH acidity.
• The malononitrile-based probe (NPI-N2) exhibited significant color change (colorless to red-violet) and fluorescence quenching with a pKa of 3.08.
• NPI-N2 demonstrated selectivity in the presence of metal ions, anions, and pepsin.
• Incorporating ester-nitrile (NPI-EN) and diethyl ester (NPI-E2) subunits modulated the pKa to 6.73 and 10.76, respectively.
• NMR studies elucidated the pH-signaling mechanism involving deprotonation of the malononitrile moiety.
• NPI-N2 was successfully applied in a paper-based test strip for artificial gastric fluid pH measurement.

Conclusions:

• Novel naphthalimide malonate derivatives serve as effective colorimetric pH sensors.
• NPI-N2 exhibits excellent pH-sensing performance and selectivity, making it suitable for biological applications.
• The developed paper-based test strip offers a convenient method for pH determination in artificial gastric fluid.