DDAB films

P. Yang, Q. Zhao, Z. Gu, and Q. Zhuang, The electrochemical behavior of hemoglobin on SWNTs/ DDAB film modified glassy carbon electrode. Electroanalysis 16, 97—100 (2004). 

An STM probe has been used to isolate individual MS (M = Cd, Pb) particles and to measure electronic phenomena (55,56,81). The MS films were prepared either by exposure of metal ion/fatty acid films to H2S (55,56) or by transfer of a compressed DDAB-complexed CdS monolayer (81). All the films were transferred onto highly oriented pyrolytic graphite (HOPG) for the STM measurements. A junction was created at an individual CdS particle with the STM tip as one electrode and the graphite as the other, and the current/voltage characteristics of the panicles were measured. For the particle prepared in the fatty acid films the I/V curves exhibit step-like features characteristic of monoelectron phenomena. In the case of the DDAB-coated CdS particles the I/V measurements demonstrated n-type semiconductor behavior. The absence of steps in this system is probably a reflection of the larger size of the particles in the DDAB films (8 nm by AFM) compared to the 2-nm particle size typically found for MS particles formed in fatty acid films. 

Fig. 14.39. Cyclic voltammograms at 100 mV s-1 in pH 5.5 buffer, a, pH 5.5 buffer on bare pyrolytic graphite (PG) b, 0.5 mM MB (horse, from Sigma) in buffer on bare PG c, MB-DDAB film on PG in buffer, no MB in solution. (Reprinted from J. F. Rusting, Electrochemical Enzyme Catalysis, Interface, 6(4) 26-31, Fig. 2, 1997. Reproduced with permission from the Electrochemical Society, Inc.)...

Figure 4.9. RDE voltammograms response of Mb/DDAB film modified GCE around the Pglll/Il couple in the presence of various nitrite concentrations. Curve (a) 2.0 mM (b) 4.0 mM (c) 6.0 mM (d) 8.0 mM. Electrolyte phosphate buffer solution, 50 mM NaBr (pH = 7.4). Rotation rate 500 rpm scan rate 0.02 Vs ...

Chen, S. and C. Tseng (2004). The characterization and bioelectrocatalytic properties of hemoglobin by direct electrochemistry of DDAB film modified electrodes. Electrochimica Acta 49, 1903-1914. 

The immobilization of P450 BM3 within di-dodecyldimethylammoniirm bromide (DDAB) films provided a very favorable envirorunent for transferring electrons from the electrode to the heme iron. This transfer was measitred directly and occurred at a fast rate ( /=221 s ), similar to the natural biological rate measitred with palmitic acid as substrate. Furthermore, the electron transfer very much depended on the nature of the substrate and showed a lower A / value of 130 s when the less favored substrate lauric acid was used [223]. 

CV peaks for Mb-DDAB films decreased less than 20% upon storage of the electrode in buffer for a month. A bare PG electrode placed into buffer or a myoglobin solution gives no peaks (Figs. 4a, b). 

The shape of the CV for this rather thick Mb-DDAB film reflects diffusion-controlled electrochemistry. The peaks are not symmetric (Fig. 4c), and have a characteristic diflfusional tail . Furthermore, peak current is proportional to the square root of scan rate, as predicted by linear diffusion theory [29, 30]. 

CVs of Mb-phosphatidylcholine films (Fig. 5) have more symmetric peak shapes, and equal reduction and oxidation peak heights which were linear functions of scan rate. These characteristics indicate thin layer electrochemical behavior [29-31] in which all electroactive MbFe(III) in the films is converted to MbFe(Il) on the forward CV scan. Unlike Mb-DDAB films, those made with phosphatidylcholines became thinner after soaking in buffer for about 1 h, but then remained stable. Thinning is a result of the slight solubility of phosphatidylcholines in water. 

Fig. 6 Influence of temperature on limiting current from normal pulse voltammetry (pulse width 10 ms) for Mb-DDAB films on PG in pH 5.5 buffer (adapted from Ref. [19])...

CVs of cyt P450 films treated with excess carbon monoxide CO showed that the midpoint potential of the CVs (Era) shifted +61 mV in DMPC films (Fig 7b2) and + 45 mV in DDAB films. After removal of CO, a CV (Fig. 7b3) nearly identical to the reversible CV of Cyt P450cam (cf- Pig 7bl) was found. Since the cyt P450Fe(II)-CO complex of rabbit liver cyt P450 has a formal potential about 150 mV positive of that of P450Fe(II) [39], the shift in the position of CVs of films in the presence of CO suggests the formation of Cyt P450camFe(II)-CO. 

Angular-dependent reflectance FT-IR analysis of the DDAB CH stretching and bending bands of Mb-DDAB films provided an estimate of the tilt angle of the hydrocarbon chains. A tilt angle of about 30° with respect to the film normal was found [19]. This is similar to tilt angles of phospholipid bilayers in crystals [7], and provides secondary evidence for surfactant ordering in the films. 

Fig. 10 Steady-state cyclic voltammograms at 100 mV s in pH 5.5 buffer of Mb-DDAB films with trichloroacetic acid (TCA) at 0,2.5,5, 7.5, and 10 mM (adapted from Ref. [54]). The direct reduction peak for TCA on a DDAB-PG electrode is found at about —1.4 V vs. SCE (not shown)...

A cyclic voltammogram of a Mb-DDAB film shows large increases in the peak current for reduction of MbFe(III) when increasing amounts of trichloroacetic acid are added to the solution (Fig. 10). Reduction occurs at potentials more than 1 V positive of the corresponding direct reductions at bare electrodes. These results are... 

Mb can be taken up from pH 5.5-7 buffers into cast liquid crystal films of didodecyldimethylammonium bromide (DDAB) on electrodes [17]. We observed chemically reversible voltammetry for the heme Fe /Fe redox couple of Mb taken up from solution into 0.5-20 [tm DDAB films (Figure 3) [17-19]. No vol-tammetric peaks were observed for bare pyrolytic graphite (PG) electrodes in Mb solutions, so the surfactant film turns on the protein electrochemistry. Mb-DDAB films retain more than 80% of the original steady state voltammetric peak current after a month s storage in buffer. DDAB and Mb are both positive at pH 5.5, so hydrophobic interactions are probably important for film stability. 

The CV peaks for 20 pm Mb-DDAB films (Figure 3) have a characteristic diffusional tail consistent with the influence of diffusion. Peak separations were about 100 mV at 0.1 V s , and peak current Ip was proportional to the square root of scan rate (v) consistent [21,22] with diffusion-controlled film voltammetry. Analysis of the linear Ip vs Vi plot gave a charge transport diffusion coeffi-... 

Figure 3 Cyclic voltammograms at 100 mV s" and 25°C (a) pH 5.5 buffer containing no protein on a bare PG electrode (b) 25 i,M Mb purified by ultrafiltration in buffer on bare PG (c) 20 0,m Mb-DDAB film on PG electrode in buffer, no Mb in solntion. (Adapted from Ref. 17 with permission. Copyright 1995 American Chemical Society.)...

DDAB films are in a lamellar liquid crystal phase at 25 C. The fluidity of this phase facilitates movement of the protein during voltammetry. In thick films, normal pulse voltammetry (NPV) limiting currents, a direct measure of [11,17] gave small NPV limiting currents for Mb-DD AB films at temperatures below the gel-to-Iiquid crystal phase transition temperature (T. ), where they are... 

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