Dabcyl as a Naked Eye Colorimetric Chemosensor for Palladium Detection in Aqueous Medium

Abstract

Industrial activity has raised significant concerns regarding the widespread pollution caused by metal ions, contaminating ecosystems and causing adverse effects on human health. Therefore, the development of sensors for selective and sensitive detection of these analytes is extremely important. In this regard, an azo dye, Dabcyl 2, was synthesised and investigated for sensing metal ions with environmental and industrial relevance. The cation binding character of 2 was evaluated by colour changes as seen by the naked eye, UV-Vis and ¹H NMR titrations in aqueous mixtures of SDS (0.02 M, pH 6) solution with acetonitrile (99:1, v/v). Out of the several cations tested, chemosensor 2 had a selective response for Pd²⁺, Sn²⁺ and Fe³⁺, showing a remarkable colour change visible to the naked eye and large bathochromic shifts in the UV-Vis spectrum of 2. This compound was very sensitive for Pd²⁺, Sn²⁺ and Fe³⁺, with a detection limit as low as 5.4 × 10^-8 M, 1.3 × 10^-7 M and 5.2 × 10^-8 M, respectively. Moreover, comparative studies revealed that chemosensor 2 had high selectivity towards Pd²⁺ even in the presence of other metal ions in SDS aqueous mixtures.

Keywords: Dabcyl; azo dye; colorimetric chemosensor; iron; naked eye; palladium; tin.

Scheme 1

Synthetic route to Dabcyl 2.

Figure 1

Colorimetric responses of 2 in SDS (0.02 M, pH 6)–acetonitrile 99:1 (v/v) solution (2 × 10⁻⁵ M) before and after the addition of 10 equiv. of various ions.

Figure 2

UV-Vis spectra of 2 in SDS (0.02 M, pH 6)–acetonitrile 99:1 (v/v) solution (2 × 10⁻⁵ M) before and after the addition of 10 equiv. of Ag⁺, K⁺, Li⁺, Na⁺, Cu⁺, TBT⁺, Hg²⁺, Ca²⁺, Co²⁺_, Pb²⁺, Mn²⁺, Fe²⁺, Zn²⁺, Ni²⁺, Cd²⁺, Cu²⁺, Pd²⁺_, Cs²⁺, Sn²⁺, Fe³⁺ and Al³⁺.

Figure 3

Absorption spectral changes of 2 with Pd²⁺ in SDS (0.02 M, pH 6)–acetonitrile 99:1 (v/v) solution (2 × 10⁻⁵ M): (a) UV-Vis titration of 2 upon gradual addition of Pd²⁺; (b) absorbance at 462 and 555 nm as a function of added Pd²⁺ equiv.; (c) Job’s plot for the complexation of 2 with Pd²⁺ at 555 nm.

Figure 4

Partial ¹H NMR spectra of 2 with the addition of Pd²⁺ (5 equiv.) in DMSO-d₆ at 25 and 60 °C (* indicates the new signals of the formed complex).

Figure 5

Suggested structure for complex 3 in DMSO solution.

Figure 6

Absorption spectra of 2 in SDS (0.02 M, pH 6)–acetonitrile 99:1 (v/v) solution (2 × 10⁻⁵ M) after 7 days at different concentrations of Pd²⁺ (A: 0 equiv.; B: 0.5 equiv.; C: 1 equiv.; D: 2 equiv.; E: 3 equiv.; F: 4 equiv.; G: 5 equiv.).

Figure 7

Suggested structure for complex 4.

Figure 8

UV-Vis spectra and colour changes of 2 in SDS (0.02 M)–acetonitrile 99:1 (v/v) solution (2 × 10⁻⁵ M) in pH levels from 2 to 10 (a) before and (b) after 5 min (full line) and 48 h (broken line) upon the addition of 10 equiv. of Pd²⁺.

Figure 9

Absorbance ratio (A₅₅₅/A₄₆₂) of 2 (2 × 10⁻⁵ M) before and after the addition of 5 equiv. of Pd²⁺ in the presence of 10 equiv. background cations in SDS (0.02 M, pH 6)–acetonitrile 99:1 (v/v) solution.

Figure 10

Absorption spectral changes of 2 with Sn²⁺ in SDS (0.02 M, pH 6)–acetonitrile 99:1 (v/v) solution (2 × 10⁻⁵ M): (a) UV-Vis titration of 2 upon gradual addition of Sn²⁺; (b) absorbance at 462 and 515 nm as a function of added Sn²⁺ equiv.; (c) Job’s plot for the complexation of 2 with Sn²⁺ at 515 nm.

Figure 11

Absorption spectral changes of 2 with Fe³⁺ in SDS (0.02 M, pH 6)–acetonitrile 99:1 (v/v) solution (2 × 10⁻⁵ M): (a) UV-Vis titration of 2 upon gradual addition of Fe³⁺; (b) absorbance at 462 and 515 nm as a function of added Fe³⁺ equiv.; (c) Job’s plot for the complexation of 2 with Fe³⁺ at 515 nm.

Figure 12

Partial ¹H NMR spectra of 2 with Sn²⁺ in DMSO-d₆ at 25 °C (* indicates the new signals).

Figure 13

Absorbance ratio (A₅₁₅/A₄₆₂) of 2 (2 × 10⁻⁵ M) before and after the addition of 5 equiv. of Sn²⁺ in the presence of 10 equiv. background metal ions in SDS (0.02 M, pH 6)–acetonitrile 99:1 (v/v) solution.

Figure 14

Absorbance ratio (A₅₁₅/A₄₆₂) of 2 (2 × 10⁻⁵ M) before and after the addition of 5 equiv. of Fe³⁺ in the presence of 10 equiv. background metal ions in SDS (0.02 M, pH 6)–acetonitrile 99:1 (v/v) solution.