A Novel Bisquaternary Ammonium Compound as an Anion Sensor-ESI-MS and Fluorescence Study

Abstract

Electrospray ionization mass spectrometry (ESI-MS) analysis is frequently associated with noncovalent adduct formation, both in positive and negative modes. Anion binding and sensing by mass spectrometry, notably more challenging compared to cation binding, will have major research potential with the development of appropriate sensors. Here, we demonstrated identification of stable bisquaternary dication adducts with trifluoroacetate (TFA^-), Cl^- and HSO₄^- in positive-mode ESI-MS analysis. The observed adducts were stable in MS/MS mode, leading to the formation of characteristic fragment ions containing a covalently bound anion, which requires bond reorganization. This phenomenon was confirmed by computational methods. Furthermore, given that anion detection and anion sensor chemistry have gained significant prominence in chemistry, we conducted an analysis of the fluorescent properties of bisquaternary ammonium compound as a potential anion sensor.

Keywords: ESI-MS; TFA anion; anion sensor; computational analysis; fluorescence; quaternary ammonium salts.

Figure 1

Schematic presentation of the synthesis of (TPP)2-CYSTAM. TEA-N,N,N-triethylamine.

Figure 2

ESI-MS spectrum of (TPP)2-CYSTAM in positive mode. m/z range from 100 to 1200.

Figure 3

The ESI-MS spectra of (A) TFA-(TPP)2-CYSTAM adduct, (B) Cl-(TPP)2-CYSTAM and (C) HSO₄-(TPP)2-CYSTAM in positive mode. m/z range from (A) 100 to 1000; (B,C) from 200 to 1000.

Figure 4

ESI-MS/MS/MS spectrum of TFA-(TPP)2-CYSTAM adduct. Parent ion 540.120 m/z; collision energy 50% m/z; range from 50 to 700.

Figure 5

The structure of the TFA-(TPP)2-CYSTAM ion (M1⁺).

Figure 6

The structure of the [M1a]⁺, [M1b]⁺ and [M1c]⁺ ions (daughter ions of TFA-(TPP)2-CYSTAM); [M2a]⁺ and [M2b]⁺ (daughter ions of Cl-(TPP)2-CYSTAM) and [M3a]⁺ and [M3b]⁺ (daughter ions of HSO₄⁻(TPP)2-CYSTAM).

Figure 7

UV–Vis spectrum of (TPP)2-CYSTAM and its adducts with TFA, Cl⁻ and HSO₄⁻.All samples were dissolved in MeOH. Panel (A): concentration of each sample was equal to 1.37 × 10⁻³. Panel (B): concentration of each sample was equal to 1.37 × 10⁻⁴ M.

Figure 8

(A) Excitation and (B) emission spectra of (TPP)2-CYSTAM and its adducts with TFA⁻, Cl⁻ and HSO₄⁻. Concentration of (TPP)2-CYSTAM = 1.37 × 10⁻⁵ M. Excitation was monitored at the emission wavelengths of 363 and 454 nm for (TPP)2-CYSTAM, 363 and 450 nm for T TFA-(TPP)2-CYSTAM, 358 and 454 nm for Cl-(TPP)2-CYSTAM and 350 and 450 nm for HSO₄-(TPP)2-CYSTAM. The wavelengths of the excitation radiation λ = 266 and 313 nm for (TPP)2CYSATM, λ = 265 and 314 nm for TFA-(TPP)2-CYSTAM, λ = 266 and 315 nm for Cl-(TPP)2-CYSTAM and λ = 315 nm for HSO₄-(TPP)2-CYSTAM.

Figure 9

Emission spectra of (TPP)2-CYSTAM. Concentration: 1.37 × 10⁻⁵ M. λ_exc for a = 313, b = 330 and c = 266 nm.

Figure 10

Excitation spectra of (TPP)2-CYSTAM. Concentration 1.37 × 10⁻⁵ M. λ_mon for a = 363 nm and b = 450 nm.

Figure 11

Emission spectra of (TPP)2-CYSTAM and its adducts with TFA⁻, Cl⁻ and HSO₄⁻. Concentration of (TPP)2-CYSTAM = 1.37 × 10⁻⁵ M. For each sample, λ_exc = 313.5 nm.

Figure 12

Excitation spectra of (TPP)2-CYSTAM and its adducts with TFA⁻, Cl⁻ and HSO₄⁻. Concentration of (TPP)2-CYSTAM = 1.37 × 10⁻⁵ M. For each sample, λ_em = 360 nm.

Figure 13

Excitation spectra of (TPP)2-CYSTAM and its adducts with TFA⁻, Cl⁻ and HSO₄⁻. Concentration of (TPP)2-CYSTAM = 1.37 × 10⁻⁵ M. For each sample, λ_em = 450 nm.

Figure 14

Emission spectra of a (TPP)2-CYSTAM depending on the amount of TFA. Excitation wavelengths: (A) 330 nm, (B) 358 nm and (C) 408 nm.

Figure 15

Excitation spectra of a (TPP)2-CYSTAM depending on the amount of TFA. Excitation spectra monitored at an emission wavelength of (A) 466 nm; (B) for TFA amount 1–0.0005%—403 nm, for 0.0001% TFA, 0.00001% TFA and (TPP)2-CYSTAM—366 nm.