<p> ELECTRONIC SUPPLEMENTARY MATERIAL</p><p>Nitrogen-doped multi-walled carbon nanotubes modified with platinum, palladium, rhodium, and silver nanoparticles in electrochemical sensing</p><p>Nikos G. Tsierkezos1• Shereen Haj Othman1• Uwe Ritter • </p><p>Lars Hafermann2• Andrea Knauer2• J. Michael Köhler2</p><p>Fig. S1 Diagram showing the size distribution for RhNPs nanoparticles (mean diameter: 2.72 nm; FWHM: 1.7 nm; total number: 3.4×1013 mL-1) </p><p>Fig. S2 Diagram showing the size distribution for PdNPs nanoparticles (mean diameter: 2.64 nm; FWHM: 1.4 nm; total number: 2.4×1013 mL-1)</p><p>1N. G. Tsierkezos () ∙ S. Haj Othman ∙ U. Ritter Department of Chemistry Institute of Chemistry and Biotechnology Ilmenau University of Technology Weimarer Straße 25, 98693 Ilmenau, Germany E-Mail: [email protected]</p><p>2L. Hafermann ∙ A. Knauer ∙ J. M. Köhler Department of Physical Chemistry and Micro Reaction Technology Institute of Chemistry and Biotechnology Ilmenau University of Technology Gustav-Kirchhof Straße 1, 98693 Ilmenau, Germany 1 Fig. S3 Diagram showing the size distribution for PtNPs nanoparticles (mean diameter: 2.65 nm; FWHM: 1.5 nm; total number: 2.22×1013 mL-1) </p><p>Fig. S4 Diagram showing the size distribution for AgNPs nanoparticles (mean diameter: 3.38 nm; FWHM: 1.58 nm; total number: 4.91×1013 mL-1) </p><p>3-/4- Fig. S5 (a) CVs recorded for various concentrations of [Fe(CN)6] (1.0 M KCl) (from inner to outer: 0.099, 0.196, 0.415, 0.595, 0.712, 0.797, and 0.990 mM) on N-MWCNTs composite</p><p>-1 3-/4- film at the scan rate of 0.02 V∙s ; (b) Variation of oxidation peak current of [Fe(CN)6] with its concentration in the range of 0.099-0.990 mM </p><p>3-/4- Fig. S6 (a) CVs recorded for various concentrations of [Fe(CN)6] (1.0 M KCl) (from inner to outer: 0.099, 0.291, 0.415, 0.506, 0.712, 0.909, and 0.990 mM) on N-MWCNTs/RhNPs</p><p>2 composite film at the scan rate of 0.02 V∙s-1; (b) Variation of oxidation peak current of</p><p>3-/4- [Fe(CN)6] with its concentration in the range of 0.099-0.990 mM</p><p>3-/4- Fig. S7 (a) CVs recorded for various concentrations of [Fe(CN)6] (1.0 M KCl) (from inner to outer: 0.099, 0.291, 0.506, 0.595, 0.712, 0.909, and 0.990 mM) on N-MWCNTs/PdNPs composite film at the scan rate of 0.02 V∙s-1; (b) Variation of oxidation peak current of</p><p>3-/4- [Fe(CN)6] with its concentration in the range of 0.099-0.990 mM </p><p>3-/4- Fig. S8 (a) CVs recorded for various concentrations of [Fe(CN)6] (1.0 M KCl) (from inner to outer: 0.099, 0.196, 0.415, 0.506, 0.595, 0.712, and 0.990 mM) on N-MWCNTs/PtNPs composite film at the scan rate of 0.02 V∙s-1; (b) Variation of oxidation peak current of</p><p>3-/4- [Fe(CN)6] with its concentration in the range of 0.099-0.990 mM</p><p>3-/4- Fig. S9 (a) CVs recorded for various concentrations of [Fe(CN)6] (1.0 M KCl) (from inner to outer: 0.099, 0.196, 0.415, 0.506, 0.595, 0.797, and 0.990 mM) on N-MWCNTs/AgNPs composite film at the scan rate of 0.02 V∙s-1; (b) Variation of oxidation peak current of</p><p>3-/4- [Fe(CN)6] with its concentration in the range of 0.099-0.990 mM</p><p>3 3-/4- Fig. S10 (a) EIS spectra recorded for various concentrations of [Fe(CN)6] (1.0 M KCl) on N-MWCNTs composite film at the frequency range from 0.1 Hz to 100 kHz. The EIS spectra</p><p>3-/4- were recorded at the half-wave potential of [Fe(CN)6] (+0.280 V vs. Ag/AgCl). The symbols are denoted as follows: 0.099 mM (open square); 0.196 mM (open circle); 0.291 mM (open upward triangle); 0.415 mM (open downward triangle); 0.595 mM (star); 0.797 mM (plus symbol); 0.990 mM (X symbol); (b) Variation of charge transfer resistance of</p><p>3-/4- [Fe(CN)6] with its concentration in the range of 0.099-0.990 mM</p><p>3-/4- Fig. S11 (a) EIS spectra recorded for various concentrations of [Fe(CN)6] (1.0 M KCl) on N-MWCNTs/RhNPs composite film at the frequency range from 0.1 Hz to 100 kHz. The EIS</p><p>3-/4- spectra were recorded at the half-wave potential of [Fe(CN)6] (+0.280 V vs. Ag/AgCl). The symbols are denoted as follows: 0.099 mM (open square); 0.196 mM (open circle); 0.291 mM (open upward triangle); 0.415 mM (open downward triangle); 0.595 mM (star); 0.797 mM (plus symbol); 0.990 mM (X symbol); (b) Variation of charge transfer resistance of</p><p>3-/4- [Fe(CN)6] with its concentration in the range of 0.099-0.990 mM</p><p>4 3-/4- Fig. S12 (a) EIS spectra recorded for various concentrations of [Fe(CN)6] (1.0 M KCl) on N-MWCNTs/PdNPs composite film at the frequency range from 0.1 Hz to 100 kHz. The EIS</p><p>3-/4- spectra were recorded at the half-wave potential of [Fe(CN)6] (+0.280 V vs. Ag/AgCl). The symbols are denoted as follows: 0.099 mM (open square); 0.291 mM (open circle); 0.415 mM (open upward triangle); 0.506 mM (open downward triangle); 0.595 mM (star); 0.797 mM (plus symbol); 0.990 mM (X symbol); (b) Variation of charge transfer resistance of</p><p>3-/4- [Fe(CN)6] with its concentration in the range of 0.099-0.990 mM</p><p>3-/4- Fig. S13 (a) EIS spectra recorded for various concentrations of [Fe(CN)6] (1.0 M KCl) on N-MWCNTs/PtNPs composite film at the frequency range from 0.1 Hz to 100 kHz. The EIS</p><p>5 3-/4- spectra were recorded at the half-wave potential of [Fe(CN)6] (+0.280 V vs. Ag/AgCl). The symbols are denoted as follows: 0.099 mM (open square); 0.196 mM (open circle); 0.291 mM (open upward triangle); 0.415 mM (open downward triangle); 0.506 mM (star); 0.712 mM (plus symbol); 0.990 mM (X symbol); (b) Variation of charge transfer resistance of</p><p>3-/4- [Fe(CN)6] with its concentration in the range of 0.099-0.990 mM</p><p>3-/4- Fig. S14 (a) EIS spectra recorded for various concentrations of [Fe(CN)6] (1.0 M KCl) on N-MWCNTs/AgNPs composite film at the frequency range from 0.1 Hz to 100 kHz. The EIS</p><p>3-/4- spectra were recorded at the half-wave potential of [Fe(CN)6] (+0.280 V vs. Ag/AgCl). The symbols are denoted as follows: 0.099 mM (open square); 0.196 mM (open circle); 0.415 mM (open upward triangle); 0.506 mM (open downward triangle); 0.595 mM (star); 0.797 mM (plus symbol); 0.990 mM (X symbol); (b) Variation of charge transfer resistance of</p><p>3-/4- [Fe(CN)6] with its concentration in the range of 0.099-0.990 mM</p><p>6 Fig. S15 Equivalent electrical circuit model (Rs+(Cdl/Rct+Zw)) used for the simulation of</p><p>3-/4- electrochemical impedance spectra recorded for [Fe(CN)6] (1.0 M KCl) on N- MWCNTs/MNPs (M: Pt, Pd, Rh, Ag) composite films (software Thales, version 4.15). The elements of electrical circuit are denoted as follows: Rs represents the resistance of electrolyte,</p><p>Rct symbolizes the charge transfer resistance of redox reaction, Cdl represents the double-layer capacitance (constant phase element), and Zw is the Warburg diffusion impedance </p><p>Fig. S16 Histograms showing the charge transfer resistance (a) and the heterogeneous</p><p>3-/4- electron-transfer rate constant (b) of [Fe(CN)6] (1.0 M KCl) on N-MWCNTs (symbolized 7 in diagram as CNT) and N-MWCNTs/MNPs (M: Rh, Pd, Pt, Ag) composite films (symbolized in diagram as Rh, Pd, Pt, and Ag)</p><p>8</p>