Solvent Effects in Halogen and Hydrogen Bonding Mediated Electrochemical Anion Sensing in Aqueous Solution and at Interfaces

Abstract Sensing anionic species in competitive aqueous media is a well‐recognised challenge to long‐term applications across a multitude of fields. Herein, we report a comprehensive investigation of the electrochemical anion sensing performance of novel halogen bonding (XB) and hydrogen bonding (HB) bis‐ferrocene‐(iodo)triazole receptors in solution and at self‐assembled monolayers (SAMs), in a range of increasingly competitive aqueous organic solvent media (ACN/H2O). In solution, the XB sensor notably outperforms the HB sensor, with substantial anion recognition induced cathodic voltammetric responses of the ferrocene/ferrocenium redox couple persisting even in highly competitive aqueous solvent media of 20 % water content. The response to halides, in particular, shows a markedly lower sensitivity to increasing water content associated with a unique halide selectivity at unprecedented levels of solvent polarity. The HB sensor, in contrast, generally displayed a preference towards oxoanions. A significant surface‐enhancement effect was observed for both XB/HB receptive films in all solvent systems, whereby the HB sensor generally displayed larger responses towards oxoanions than its halogen bonding analogue.


S1.1 General Information:
All experiments were completed at room temperature in the presence of oxygen, unless stated otherwise. All chemicals and solvents were obtained from commercial suppliers and used as received unless noted otherwise. Supporting electrolyte (TBAClO4 from Sigma Aldrich) was of electrochemical grade. All hygroscopic TBA salts were stored in vacuum dessicators at room temperature. Anhydrous solvents were degassed with N2 and dried on a Mbraun MPSP-800 column. Ultrapure water was drawn from a Milli-Q system (18.2 MΩcm). Electrochemical measurements were performed with an Autolab Potentiostat (Metrohm). All measurements were carried out in a 3-electrode cell equipped with a Au disc (or glassy carbon disc electrode for studies under diffusive conditions, BASi 3 mm diameter) working electrode (BASi or IJ Cambria; 1.6 or 2 mm diameter), a platinum wire counter electrode and either a Ag|AgCl (3 M KCl) or non-aqueous Ag|AgNO3 reference electrode, depending on the solvent used. Unless stated otherwise, all potentials are wrt. non-aqueous Ag|AgNO3 reference electrode. Mass spectrometry was performed on a Bruker micrOTOF. NMR spectra were recorded on Bruker NMR spectrometers (AVIII HD 500 or AVIII HD 400)).

S1.2 Electrochemical Measurements
Electrochemical measurements were performed with an Autolab Potentiostat (Metrohm). All measurements were carried out in a 3-electrode cell equipped with a Au disc working electrode for interfacial studies (BASi or IJ Cambria; 1.6 or 2 mm diameter), or glassy carbon disc electrode for studies under diffusive conditions (BASi 3 mm diameter), a platinum wire counter electrode and either a Ag|AgCl (3 M KCl) or non-aqueous Ag|AgNO3 reference electrode, depending on the solvent used. Unless stated otherwise, all potentials are wrt.
non-aqueous Ag|AgNO3 reference electrode. The half-wave potentials were determined as the peak potential by SWV. 4

S1.3 Electrode Cleaning Procedure
Au disc electrodes and glassy carbon electrode were polished mechanically with a 0.05 μm alumina slurry for 2 min, followed by sonication in 1:1 EtOH : H2O for several minutes. The Au disc electrodes were then chemically polished by submersion in fresh piranha acid (3:1 conc. H2SO4: 30% H2O2), and subsequently electrochemically polished in 0.5 M KOH from -0.7 V to -1.7 V, then in 0.5 M H2SO4 from -0.15 V to 1.35 V for at least 1 hour each (at a scan rate of 100 mV s -1 , potentials wrt to aqueous Ag|AgCl (3 M KCl) reference electrode).

S1.4 SAM Formation
Immediately following electrochemical polishing, the Au disc electrodes were rinsed thoroughly with copious amounts of water and ethanol, dried under a stream of N2 and submerged in the receptor solution. Formation of SAMs of the receptors was carried out by immersion of the Au disc electrodes in 1 mM 1.XB/HB solutions in DCM overnight in the dark, followed by thorough washing with DCM, yielding 1.XB/HBSAM.

S1.5 Surface characterisation:
SAMs for ellipsometry or IR analyses were formed in the same manner as on gold disc electrodes (1 mM receptor in DCM) but on larger gold on silicon substrates (prepared inhouse), which were cleaned by immersion in fresh piranha solution (3:1 conc. H2SO4: 30% H2O2) and copious rinsing with EtOH and H2O.
FT-ATR-IR spectra were measured on an IRTracer-100 (Shimadzu). Water contact angles measurements were recorded with a FTA1000B goniometer (First Ten Ångstroms). All ellipsometry measurements were performed on a Beaglehole Instruments Picometer equipped with a 2 mW Helium-Neon laser (632.8 nm) and average film thicknesses and standard deviations were calculated from five different measurements on the same substrate. 5 S1.6 Capacitance Measurements: The film capacitance, C was obtained by electrochemical impedance spectroscopy (EIS) in 100 mM NaPF6 (aq) vs. Ag|AgCl (3 M KCl) reference electrode, which was recorded between 40 kHz and 0 Hz for 40 frequencies which were logarithmically stepped, using a 10 mV amplitude and 0 V as the DC potential. C was then used in conjunction with measured film thicknesses, d (as determined via ellipsometry), the surface area of the Au working electrode, A and the permittivity of free space, to calculate the dielectric constant of the film, via the relationship described by eqn S1. [1] = ( 0 × × ) (Eqn S1)

S1.7 Data Analysis and Fitting of Binding Isotherms
All data analysis and fitting was carried out with OriginPro 2017. Analysis of the sensor responses was carried out via eqns.

S3.1 Electrochemical characterisation:
A peak separation of 62 ± 1 mV and 57 ± 1 mV for of 1.XB/HBdif, respectively, and a close to unity value of the ratio of anodic and cathodic peak currents ascertained quasi-reversibility of the Fc|Fc + couple. However, the linear dependence of the peak currents on the scan rate suggests some degree of physiorption of the receptors onto the glassy carbon working electrode surface (Figures S13, S14, S15).  Figure S17. Structure of 2.XB/HBdif. [3] The general sensing properties of both 1.XB/HBdif compare favourably with recently reported Fc-isophthalamide-(iodo)triazole receptors 2.XB/HBdif (Figures S18 and S19). [3] For example, as shown in Figure S18A Table S1. Tabulated values from linear fits shown in Figure S23 and S24 (slopes and R-square values) for plots of ΔEmax vs. water content from titrations of various anions with 1.XB/HBSAM/dif in all solvent systems. Values in parentheses denote the slopes excluding data points from measurements in ACN.

1.HBdif
The prominent peak at 2355 cm -1 is attributed to atmospheric CO2. A distinguishable peak which can be attributed to the C-I bonds in 1.XB/1.XBSAM could not be identified and is therefore expected to fall in the fingerprint region. The significant difference in film dielectric is neither expected, nor easy to rationalise. XB motifs are generally associated with a higher hydrophobicity (and lower dielectric) as previously noted in similar XB/HB receptive films. We believe that the opposite trends reported herein (in terms of both film hydrophobicity as resolved by water contact angle measurements as well as the lower dielectric, Table 2) are not reflective of the receptor chemistry itself but instead arise from other factors, such as different interfacial receptor conformations (as supported by moderate discrepancies in receptor surface coverage and film thickness, Table 2