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Tuning the isoelectric point of graphene by electrochemical functionalization.

Zuccaro L, Krieg J, Desideri A, Kern K, Balasubramanian K - Sci Rep (2015)

Bottom Line: The ability to control the charge-potential landscape at solid-liquid interfaces is pivotal to engineer novel devices for applications in sensing, catalysis and energy conversion.The isoelectric point (pI)/point of zero charge (pzc) of graphene plays a key role in a number of physico-chemical phenomena occurring at the graphene-liquid interface.The pI of bare graphene (as-prepared, chemical vapor deposition (CVD)-grown) is found to be less than 3.3, which we can continuously modify up to 7.5 by non-covalent electrochemical attachment of aromatic amino groups, preserving the favorable electronic properties of graphene throughout.

View Article: PubMed Central - PubMed

Affiliation: Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany.

ABSTRACT
The ability to control the charge-potential landscape at solid-liquid interfaces is pivotal to engineer novel devices for applications in sensing, catalysis and energy conversion. The isoelectric point (pI)/point of zero charge (pzc) of graphene plays a key role in a number of physico-chemical phenomena occurring at the graphene-liquid interface. Supported by theory, we present here a methodology to identify the pI/pzc of (functionalized) graphene, which also allows for estimating the nature and extent of ion adsorption. The pI of bare graphene (as-prepared, chemical vapor deposition (CVD)-grown) is found to be less than 3.3, which we can continuously modify up to 7.5 by non-covalent electrochemical attachment of aromatic amino groups, preserving the favorable electronic properties of graphene throughout. Modelling all the observed results with detailed theory, we also show that specific adsorption of ions and the substrate play only an ancillary role in our capability to tune the pI of graphene.

No MeSH data available.


The isoelectric point of functionalized graphene for 3 nm-thick layers of pABA (a,b) and pANI (c,d).(a) Measured and calculated Dirac point profiles for Gr/pABA as a function of pH at 1 mM and 1 M ionic strength. (b) Difference Dirac curves (obtained by subtracting the two curves in (a) before (black) and after modification (blue) giving an estimate of IEP as around 6 for Gr/pABA. (c) Measured and calculated Dirac point profiles for Gr/pANI as a function of pH at 1 mM and 1 M ionic strength. (d) Difference Dirac curves before (black) and after modification (red) giving an estimate of IEP to be around 4. The pI of bare graphene is less than 3.3 in both cases. See supplementary information about the details of model parameters (Figs S6–S8).
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f4: The isoelectric point of functionalized graphene for 3 nm-thick layers of pABA (a,b) and pANI (c,d).(a) Measured and calculated Dirac point profiles for Gr/pABA as a function of pH at 1 mM and 1 M ionic strength. (b) Difference Dirac curves (obtained by subtracting the two curves in (a) before (black) and after modification (blue) giving an estimate of IEP as around 6 for Gr/pABA. (c) Measured and calculated Dirac point profiles for Gr/pANI as a function of pH at 1 mM and 1 M ionic strength. (d) Difference Dirac curves before (black) and after modification (red) giving an estimate of IEP to be around 4. The pI of bare graphene is less than 3.3 in both cases. See supplementary information about the details of model parameters (Figs S6–S8).

Mentions: Figure 4(a) presents the pH-dependent Dirac point behavior of a graphene device at two different ionic strength values (1 mM and 1 M) after attachment of 3 nm-thick functional layers of poly(aminobenzylamine) (pABA). Figure 4(b) shows the M-I difference Dirac curve (blue) obtained from this measurement along with the difference Dirac curve before modification (black curve). The curve before modification does not have a zero crossing consistent with the discussion that the pI/pzc of graphene is less than 3.3. However, after modification the difference curve exhibits a clear zero crossing at around a pH of 6, which is attributed to be the approximate pI of the functionalized surface. In order to obtain support for this claim, we have used the model to simulate the presence of pABA by introducing an additional type of ionizable group with a charge density that is around 1.2 times that of graphene ( for this device) and a pKa of 8.5. The effect of the silanol groups at (15%) and that of specific ion adsorption (7%) remain the same as for bare graphene. The fitted curves are presented as solid lines in Fig. 4(a) where it can be seen that a qualitatively good agreement is obtained. From this model we extract a pI of 5.98 and pzcs of 5.98 and 5.6 at an IS of 1 mM and 1 M respectively, while the experimental zero crossing of M-I difference curve occurs at 5.92. Further support for this estimation is obtained from Fig. S6 presenting the complete dataset, along with the simulated curves for all the 4 IS values. Almost all the aspects, including the shift in the zero crossing (which can be unambiguously attributed to specific adsorption of anions) are well-reproduced using the model. In order to confirm that we are indeed modifying the pI of the surface we have repeated the same measurement for the case of polyaniline (pANI). The theoretical curves are modelled by setting the pKa of the type of ionizable group to 4.2 (instead of 8.5 as done before). The results collected in Fig. 4(c,d) (see also Fig. S7), show a clear agreement between theory and experiment. The model gives a pI value of 3.85 and pzcs of 3.82 and 3.38 at 1 mM and 1 M IS respectively, while the experimental M-I zero crossing occurs at 4.05. Further details of the model parameters are discussed in supplementary information. Based on these observations, we will now use the pH value of zero crossing of the M-I difference Dirac curve directly as an approximate estimate of the surface pI.


Tuning the isoelectric point of graphene by electrochemical functionalization.

Zuccaro L, Krieg J, Desideri A, Kern K, Balasubramanian K - Sci Rep (2015)

The isoelectric point of functionalized graphene for 3 nm-thick layers of pABA (a,b) and pANI (c,d).(a) Measured and calculated Dirac point profiles for Gr/pABA as a function of pH at 1 mM and 1 M ionic strength. (b) Difference Dirac curves (obtained by subtracting the two curves in (a) before (black) and after modification (blue) giving an estimate of IEP as around 6 for Gr/pABA. (c) Measured and calculated Dirac point profiles for Gr/pANI as a function of pH at 1 mM and 1 M ionic strength. (d) Difference Dirac curves before (black) and after modification (red) giving an estimate of IEP to be around 4. The pI of bare graphene is less than 3.3 in both cases. See supplementary information about the details of model parameters (Figs S6–S8).
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
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f4: The isoelectric point of functionalized graphene for 3 nm-thick layers of pABA (a,b) and pANI (c,d).(a) Measured and calculated Dirac point profiles for Gr/pABA as a function of pH at 1 mM and 1 M ionic strength. (b) Difference Dirac curves (obtained by subtracting the two curves in (a) before (black) and after modification (blue) giving an estimate of IEP as around 6 for Gr/pABA. (c) Measured and calculated Dirac point profiles for Gr/pANI as a function of pH at 1 mM and 1 M ionic strength. (d) Difference Dirac curves before (black) and after modification (red) giving an estimate of IEP to be around 4. The pI of bare graphene is less than 3.3 in both cases. See supplementary information about the details of model parameters (Figs S6–S8).
Mentions: Figure 4(a) presents the pH-dependent Dirac point behavior of a graphene device at two different ionic strength values (1 mM and 1 M) after attachment of 3 nm-thick functional layers of poly(aminobenzylamine) (pABA). Figure 4(b) shows the M-I difference Dirac curve (blue) obtained from this measurement along with the difference Dirac curve before modification (black curve). The curve before modification does not have a zero crossing consistent with the discussion that the pI/pzc of graphene is less than 3.3. However, after modification the difference curve exhibits a clear zero crossing at around a pH of 6, which is attributed to be the approximate pI of the functionalized surface. In order to obtain support for this claim, we have used the model to simulate the presence of pABA by introducing an additional type of ionizable group with a charge density that is around 1.2 times that of graphene ( for this device) and a pKa of 8.5. The effect of the silanol groups at (15%) and that of specific ion adsorption (7%) remain the same as for bare graphene. The fitted curves are presented as solid lines in Fig. 4(a) where it can be seen that a qualitatively good agreement is obtained. From this model we extract a pI of 5.98 and pzcs of 5.98 and 5.6 at an IS of 1 mM and 1 M respectively, while the experimental zero crossing of M-I difference curve occurs at 5.92. Further support for this estimation is obtained from Fig. S6 presenting the complete dataset, along with the simulated curves for all the 4 IS values. Almost all the aspects, including the shift in the zero crossing (which can be unambiguously attributed to specific adsorption of anions) are well-reproduced using the model. In order to confirm that we are indeed modifying the pI of the surface we have repeated the same measurement for the case of polyaniline (pANI). The theoretical curves are modelled by setting the pKa of the type of ionizable group to 4.2 (instead of 8.5 as done before). The results collected in Fig. 4(c,d) (see also Fig. S7), show a clear agreement between theory and experiment. The model gives a pI value of 3.85 and pzcs of 3.82 and 3.38 at 1 mM and 1 M IS respectively, while the experimental M-I zero crossing occurs at 4.05. Further details of the model parameters are discussed in supplementary information. Based on these observations, we will now use the pH value of zero crossing of the M-I difference Dirac curve directly as an approximate estimate of the surface pI.

Bottom Line: The ability to control the charge-potential landscape at solid-liquid interfaces is pivotal to engineer novel devices for applications in sensing, catalysis and energy conversion.The isoelectric point (pI)/point of zero charge (pzc) of graphene plays a key role in a number of physico-chemical phenomena occurring at the graphene-liquid interface.The pI of bare graphene (as-prepared, chemical vapor deposition (CVD)-grown) is found to be less than 3.3, which we can continuously modify up to 7.5 by non-covalent electrochemical attachment of aromatic amino groups, preserving the favorable electronic properties of graphene throughout.

View Article: PubMed Central - PubMed

Affiliation: Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany.

ABSTRACT
The ability to control the charge-potential landscape at solid-liquid interfaces is pivotal to engineer novel devices for applications in sensing, catalysis and energy conversion. The isoelectric point (pI)/point of zero charge (pzc) of graphene plays a key role in a number of physico-chemical phenomena occurring at the graphene-liquid interface. Supported by theory, we present here a methodology to identify the pI/pzc of (functionalized) graphene, which also allows for estimating the nature and extent of ion adsorption. The pI of bare graphene (as-prepared, chemical vapor deposition (CVD)-grown) is found to be less than 3.3, which we can continuously modify up to 7.5 by non-covalent electrochemical attachment of aromatic amino groups, preserving the favorable electronic properties of graphene throughout. Modelling all the observed results with detailed theory, we also show that specific adsorption of ions and the substrate play only an ancillary role in our capability to tune the pI of graphene.

No MeSH data available.