<p> Electronic Supplementary Material</p><p>Highly sensitive photometric determination of cyanide based on selective etching of gold nanorods Sujin Lee1,2 ∙ Yun-Sik Nam1,2 ∙ Sung-Hee Choi1 ∙ Yeonhee Lee1,2 ∙ Kang-Bong Lee1,2,*</p><p>1Advanced Analysis Center and Green City Technology Institute, Korea Institute of Science & Technology, Hwarang-ro 14-gil 5 SeongBuk-gu, Seoul 136-791, Republic of Korea 2University of Science & Technology, 217 Gajeong-ro Yuseong-gu, Daejeon 305-350, Republic of Korea *Corresponding author. Tel.: +82 2 958 5957; fax.: +82 2 958 5810, E-mail address: [email protected] (K.-B. Lee).</p><p>Optimum conditions for CN detection</p><p>Several parameters that influenced the selectivity, sensitivity, and AuNR stability were studied to search optimum conditions for the measurement of CN.</p><p>Optimum pH The presence of chlorine ions was a prerequisite for effective etching of AuNRs, showing that composite behavior between chlorine and gold ions facilitated chemical etching of gold metal. Thus, a higher concentration of chlorine ions generally led to a larger blue-shift of the AuNR LSPR peak, corresponding to a faster etching rate [2830]. Removal of chlorine ions from the solution generally stopped the etching reaction, resulting in AuNRs with preferable aspect ratios. However, CN etching of the AuNRs was noticeably optimized at pH 6 (Fig. S1),</p><p>3+ 6+  2+ unlike that seen using other etching ions (Fe , Cr , NO2 , and Cu ), which were optimal at pH 0~1.5 (Table 2).</p><p> Hydrogen cyanide is weakly acidic, with a pKa of 9.36, and is predominant in CN solutions at low pH, according to equation (1). Although a high concentration of chlorine ions usually enables effective etching of AuNRs, in this instance a high concentration of chlorine</p><p>1 ions would instead interrupt AuNR etching because HCN would be predominant rather than CN at this low pH. Therefore, these positive and negative effects of chlorine ion concentration on efficient AuNR etching must be optimally diminished at pH 6.</p><p>  CN + H2O → HCN + OH (1)</p><p>Optimum CTAB concentration CTAB is well known to effectively absorb to the surface of AuNRs, apparently prohibiting chemical etching. In addition, the presence of CTAB also results in a great quantity of Br to greatly speed up the AuNR etching process. Since the surface area of the AuNRs is small, it can easily be loaded with CTAB. The successive increase of CTAB did not expedite further surface passivation of AuNRs but did increase the concentration of Br, thus speeding up the etching process. Hence, the AuNR etching process generally became faster with increasing CTAB concentration [2426]. However, CN etching of AuNRs was optimized with only 2 mM CTAB because, in contrast to the earlier hypothesis, the etching process was decelerated by CTAB concentrations higher than 2 mM (Fig. S2). This was because the Br concentration was increased by higher CTAB concentrations at optimum pH 6.</p><p>Optimum temperature The average aspect ratio of AuNRs in solution was previously found to decrease with increasing temperature, while the mean width remained constant [35]. These observations were known to result from the difference in thermal stability as a function of micelle length. Thermal reshaping of the CN etching reaction on AuNRs was examined in the range 2550</p><p>C, as shown in Fig. S3. Although the absorption ratio (A538/A640) increased as the temperature was raised, the system was controlled at room temperature because, at this temperature, CN etching was able to show a distinct color change for AuNRs within 30 min.</p><p>Optimum reaction time</p><p>The changes in the absorption ratio (A538/A640) of AuNRs were governed by the cyanide etching reaction. Hence, UV-Vis spectroscopy could be used to detect the end point of the reaction (Fig. S4). This was found to occur within 30 min at cyanide concentrations between 0.1 and 0.4 mM.</p><p>2 Table S1 Analytical results for the detection of CN– in real water samples obtained from tap, pond and waste water located at the Korea Institute of Science and Technology. CN content (nM) sample using the colorimetric probe Tap water < 0.5</p><p>Pond water < 0.5</p><p>Waste water < 0.5</p><p>Fig. S1 (A) Color changes of AuNR dispersions in the pH range 4–13 using 2 mM CTAB and</p><p> 0.35 mM CN incubated for 30 min. (B) Corresponding absorbance ratios (A538/A640) for AuNR dispersions at various pH values.</p><p>3 Fig. S2 (A) Photographic images of AuNR dispersion upon addition of 0.35 mM CN and CTAB surfactants at concentrations of 0–20 mM after 30 min incubation (control indicates an</p><p>AuNR dispersion without CTAB surfactant). (B) Corresponding absorbance ratios (A538/A640) for 0–5 mM CTAB surfactant.</p><p>Fig. S3 Effect of reaction temperature on AuNRs (conditions: 50.6 mM AuNRs, 0.35 mM CN, pH 6, 30 min).</p><p>4 Fig. S4 Plots of time dependent absorption ratios (A538/A640) in the presence of varying concentrations of CN ions over 70 min (inset: magnification of kinetic curve for 10 nM).</p><p>5</p>