Cationic poly(2-(methacryloyloxy)ethyl trimethylammonium chloride) (PMETAC) brushes are grown from glass slides via surface-initiated atom transfer radical polymerization (SI-ATRP). The interaction potential energy between the surface-grown polyelectrolyte brushes and a 5 μm diameter polystyrene (PS) particle in different electrolyte environments (NaCl and NaClO₄) has been directly measured using total internal reflection microscopy (TIRM). At a NaCl concentration of 0.05 mM, the measured interaction potential energy profiles show a long-range repulsion, arising from an electrical double layer interaction between the positively charged PS particle and the charges from the PMETAC layers. Upon addition of 1 mM NaCl, screening effects reduce the separation distance (h) between the PS particle and the surface, accompanied by the collapse of the PMETAC brushes. However, when the sample solution is replaced with a low concentration NaCl (0.05 mM) solution, h returns to the original values, indicating that the brushes resume the extended conformation. A remarkable difference is observed when similar TIRM experiments are carried out in the presence of NaClO₄ solution. At the same concentration or ionic strength, NaClO₄ causes a much more abrupt decrease in h between the particle and surface than that with NaCl solution. Conversely, flushing the sample cell with highly diluted NaClO₄ or pure water cannot restore the original interaction potentials, thus implying an irreversible collapse of the PMETAC brushes in the presence of ClO₄⁻ anions. This apparent change in the PMETAC brush characteristics can be associated with the strong ion-pairing interaction between the ClO₄⁻ anions and the quaternary ammonium group in the PMETAC brushes, which experiences not only pure charge screening, but also a sharp hydrophilic-to-hydrophobic change of the brush layer. Using TIRM, we have directly measured such an ion-pairing induced collapse transition of surface-confined polyelectrolyte brushes, even at very low specific anion concentrations.