Nanoscale Surface Charge Visualization of Human Hair - Analytical

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Nanoscale Surface Charge Visualization of Human Hair Faduma M. Maddar, David Perry, Rhiannon Brooks, Ashley Page, and Patrick R. Unwin Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05977 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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Analytical Chemistry

Nanoscale Surface Charge Visualization of Human Hair Faduma M. Maddar, a David Perry, a Rhiannon Brooks, a,b Ashley Page a,c and Patrick R. Unwin a,*

a Department

of Chemistry, b Molecular Analytical Science Centre for Doctoral Training,c

MOAC Doctoral Training Centre, University of Warwick, Coventry, CV4 7AL, UK

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table of Contents Graphic


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ABSTRACT The surface charge and topography of human hair are visualized synchronously at the nanoscale using scanning ion conductance microscopy (SICM), a scanning nanopipette probe technique that uses local ion conductance currents to image the physicochemical properties of interfaces. Combining SICM data with finite element method (FEM) simulations that solve for ion transport at the nanopipette under bias, allows quantitative correlation of co-located surface charge and topography. The hair samples studied herein, from a 25-year-old Caucasian male with light hair (as an exemplar), reveal that untreated hair, in areas ca. 1 cm from the root, has a fairly uniform negative charge density of ca. – 15 mC/cm-2 (in pH 6.8 aqueous solution), with some higher magnitude negative values localized near the boundaries between hair cuticles. Common chemical treatments result in varying degrees of charge heterogeneity. A bleach treatment produces some highly negatively charged localized regions (– 80 to – 100 mC/cm-2 at pH 6.8), due to hair damage, while a chemical conditioner treatment causes an overall increase in the homogeneity of the surface charge, together with a shift in the surface charge to positive values. Bleached surfaces are temporarily repaired to some extent through the use of a conditioner, as judged by the surface charge values. Finally, SICM is able to detect differences in the surface charge density of hair at different distances from the root (equivalent to hair age). Presently, the assessment of hair surface charge mainly relies on zeta-potential measurements which lack spatial resolution, among other drawbacks. In contrast, SICM enables quantitative surface charge mapping that should be beneficial in deepening understanding of the physicochemical properties of hair, and lead to the rational development of new treatments and the assessment of their efficacy at the nanoscale. Given the widespread interest in the surface charge properties of interfaces, this work further demonstrates that SICM should become an important characterization tool for surface analytical chemists generally.



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INTRODUCTION The hair care industry is fueled by a desire for products that can improve the look and feel of hair. In order to formulate better products, there is a particular interest in understanding how active ingredients deposit onto and alter the properties of hair, as this is closely related to their performance. For example, conditioners lightly coat the hair and can produce dramatic changes in the surface properties, which lead to it feeling softer and smoother.1 Numerous methods have been employed by researchers in an attempt to understand the resulting modifications and to verify efficacy.2-5 The overall goal of this paper is to introduce the use of scanning ion conductance microscopy (SICM) as a tool to quantitatively map the charge distribution on hair, following various treatments, to aid understanding of the physicochemical characteristics, among which surface charge is a key parameter. The structure of hair has been studied extensively using light, electron and atomic force microscopies, in addition to x-ray diffraction techniques.6-10 Human hair is a complex fiber (see schematic in Figure 1 (i)) that consists of various morphologic and chemical components.11 The cortex makes up the majority of the hair fiber, surrounded by the cuticle outermost layer that protects the cortex, by functioning as a barrier to chemicals and water. The cuticle consists of different sub-lamellar sheets commonly referred to as the endocuticle, the exocuticle, the Alayer, and a thin outer membrane called the epicuticle. The epicuticle comprises of a protein matrix and a lipid layer formed of fatty acids, predominantly 18-methyleicosanoic acid covalently attached through thioester linkages involving the cysteine residues of the protein, as shown in Figure 1 (ii).11,

12

Despite its importance the detailed molecular structure of the

epicuticle is poorly understood, and there is some uncertainty as to the thickness, organization and permeability of the lipid layer.13 The AFM topography map in Figure 1 (iii), shows the characteristic overlapping cuticle scales. The protein network consists of sulfur-containing


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amino acids (mostly cysteine) linked together by disulfide bonds generating cystine. These disulfide bonds play a key role in the mechanical stability of the fiber,14 while the lipid layer determines surface properties such as hydrophobicity and the low friction of untreated pristine hair.11, 12, 15-17 Surface charge is another important property that must be considered to fully understand the effect of products such as shampoos, conditioners and hair-dyeing substances. At neutral pH, untreated human hair has a negatively charged surface due to a relatively low isoelectric point (high concentration of negative sites) of 3.67.18 The negative charge arises from the sulfonate and carboxyl groups present in the proteins and lipids that constitute the hair fiber.11 The application of the aforementioned products can result in the modification of the charge presented at the hair surface.11 Surface charge properties of hair are mainly assigned from zeta-potential values, determined through streaming potential measurements.3,

19-23

However, zeta potential measurements are an average of the entire surface and, as welldocumented,24 do not necessarily provide reliable quantitative measurements of surface potential or surface charge density. A more advanced approach for obtaining local charge information has been the use of force microscopy techniques including Kelvin probe force microscopy (KPFM) and electrostatic force microscopy (EFM).23,

25, 26AFM-based

Kelvin

probe methods have been used to image both the structure and surface potential of human hair, simultaneously.27 These measurements were made in air, a basic requirement for conventional KPFM,28 with a range of humidity, for different hair treatments. However, data from KPFM are non-trivial to analyze, especially for biomaterials,28, 29 with the resulting data often affected by issues around calibration of the probe and signal drift, atmospheric contamination and moisture effects on the probe and sample (when not operated under dry conditions, which would be inappropriate for biological samples).30



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Herein, we use scanning ion conductance microscopy (SICM) to map the surface charge and topography of hair synchronously. SICM often uses a simple single barrel nanopipette for probing an interface that is bathed in a conducting electrolyte solution. In a typical set-up, the nanopipette is filled with an electrolyte solution and a quasi-reference counter electrode (QRCE) is inserted, with a second QRCE placed in the bulk electrolyte solution. A voltage bias is applied between the two QRCEs to produce an ionic current that passes through the end of the nanopipette. Changes in the ionic current as the nanopipette approaches the substrate can be used to sense and provide information about the interface.31-33 SICM has been used mainly to image the topography of surfaces, and can outperform AFM in the study of soft and living samples.34 However, it has recently been demonstrated that the ionic current responds to other interfacial properties, such as variations in surface charge.35-41 This has led to several SICM approaches that seek to map surface charge and topography synchronously.37, 38, 40, 42, 43 In particular, our group introduced the first approach to resolve topography and surface charge independently44 and has subsequently developed this to increase the speed and precision through

the

use

of

hopping

probe

mode,45,

46

combined

with

potential-pulse

chronoamperometry (current-time) measurements near the surface and in bulk at each and every pixel in an image. This allows continual self-referencing of the probe response near the surface to the bulk, enabling surface charge heterogeneities to be mapped with high sensitivity. The work in this paper demonstrates SICM as a powerful technique for quantitative charge mapping of human hair, highlighting heterogeneities in charge distribution in native (untreated hair), the significant changes that result after a bleach treatment, and further changes after the application of cosmetics (conditioner). Given that products in the hair-care industry are designed to modify hair surface chemistry, especially surface charge, SICM should be a very powerful technique to determine the efficacy and mode of action of products at the nanoscale.


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The major new insights that SICM can provide should facilitate the rational development of improved products.

Figure 1. (i) Schematic of the structure of a hair fiber which consists of three layers: the medulla, the cortex, and the cuticles. (ii) Schematic of the molecular structure of the outer hair surface consisting of a fatty acid monolayer connected to the protein complex through thioester bonds involving the cysteine residues of the protein. (iii) AFM topography map showing the characteristic overlapping structure of the cuticles that protect the cortex.



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EXPERIMENTAL Solutions For all SICM measurements carried out herein, 50 mM KCl (Sigma-Aldrich) solution, prepared using high purity water (Purite, Select HP) with a resistivity ca. 18.2 MΩ cm at 25 °C was used as the supporting electrolyte. The chemical treatments that were used are outlined below. Nanopipettes Nanopipettes with a tip diameter of 220 nm were pulled from borosilicate glass capillaries (o.d. 1.2 mm, i.d. 0.69 mm, length 100 mm, Harvard Apparatus) using a laser puller (P-2000, Sutter Instruments) with pulling parameters: Line 1: Heat 330, Fil 3, Vel 30, Del 220, Pul -; Line 2: Heat 300, Fil 3, Vel 40, Del 180, Pul 120. Two Ag/AgCl wire electrodes were used, one placed inside the nanopipette and a second placed in the bulk solution. Sample Preparation All hair samples were provided by a 25-year-old Caucasian male with light hair. Prior to analysis, hair was washed with ultra-pure water, and then boiled in a solution mixture of ethanol (≥ 99.5%) and acetone (≥ 99.9 %) in a 1:1 v/v ratio.47 Unless stated otherwise, samples were analyzed close to the root (newest growth, most pristine). The hair was then carefully dried with nitrogen (BOC). Herein, this is referred to as the untreated, clean hair. For hair treated with dye, Jerome Russell B Blonde Powder Bleach and B Blonde cream peroxide (40 Vol, 12% strength) were mixed at a ratio of 1:1 just before use. The mixture was then applied to the hair and left for 60 min at 25 °C. After applying, the hair was then rinsed with ultra-pure water and dried using nitrogen. For conditioned hair, Herbal Essences dazzling shine conditioner by Clairol was used. For full ingredients of products, see the Supporting 8 ACS Paragon Plus Environment

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Information (SI) 1, Section SI1-1. Conditioner was evenly applied to the hair and left for 60 min at 25 °C. The hair was then rinsed with ultra-pure water and dried using nitrogen. All scans, except where mentioned otherwise, were performed in an area ca. 1 cm from the root of the hair. A selected hair fiber was secured onto a glass-bottomed Petri dish (Willco Wells) leaving the surface of hair exposed for imaging. 50 mM KCl electrolyte solution was then added as a bath solution for SICM measurements. SICM Instrumentation Lateral movement of the nanopipette probe was achieved using an xy-piezoelectric positioning system with a range of 300 μm on each axis (Nano-BioS300, Mad City Laboratories, Inc.), whilst movement perpendicular to the hair substrate was controlled using a unidirectional z-piezoelectric positioner with a range of 38 μm (P-753-3CD, Physik Instrument). The electrometer and current-voltage converter used herein were both made inhouse. Instrument control and data acquisition was achieved using an FPGA card (7852R, National Instruments) with a custom-made program in LabVIEW (2013, National Instruments). The SICM set-up was mounted onto the stage of a fluorescence microscope (Axiovert 40 CFL, Zeiss) to enable precise positioning of the nanopipette over the hair substrate. All data were processed using MatLab (2017a, MathWorks). Charge mapping Surface charge maps depicted herein were collected using a self-referencing scan hopping mode of SICM shown schematically in Figure 2.40 Hopping scans were typically recorded with a resolution of 40 × 40 pixels over an area of 10 × 10 µm and consisted of the following procedure, as illustrated in the potential-time trace in Figure 2: (I) Initially, the probe 9 ACS Paragon Plus Environment


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approached the hair surface at a speed of 5 μm s-1 with the QRCE in the nanopipette probe biased at 50 mV vs. the QRCE in the bulk solution. With this small bias, the SICM current response essentially depends on the distance between the nanopipette tip and surface, with surface charge having negligible influence, as demonstrated in SI1, Section SI1-2, Figure S1.37, 40

When the ionic current between the two electrodes had decreased, with respect to the bulk

value, by a specified threshold value and hence reached the desired near-surface distance (determined from FEM simulations, see SI1, Section SI1-2), the probe movement ceased, and the corresponding z-piezo position marked the sample height at this pixel. A potential-pulse to -400 mV was then applied for a period of 50 ms (II). After this pulse, the potential bias was returned to 50 mV and the nanopipette was retracted by 2 μm away from the hair surface at 20 μm s-1 (III). The probe potential was then switched to -400 mV for a 50 ms pulse so as to record the corresponding chronoamperometric response in the bulk solution (IV) for comparison to the near-surface response. This completed the data recording at one pixel, and the probe then moved laterally (250 nm) to the next pixel at 20 μm s-1, with the nanopipette QRCE once again biased at 50 mV with respect to the bulk QRCE to repeat the process (V). The current and tip position were measured constantly throughout. AFM imaging AFM images of hair topography were recorded in air, using an Innova® AFM in tapping mode. Si-tips on nitride lever were used, with a spring constant of 0.35 N m-1 according to the manufacturer (RESP-10, Bruker).


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Figure 2. Schematic of the SICM set-up for high-speed charge mapping of hair and a trace of the z-position and potential at each hop. See text for full description. In brief, a hopping mode was used, whereby the probe was moved between bulk and near-surface (at an array of 40x40 predefined pixels. At VApproach = 50 mV (tip electrode potential), the current was sensitive to the distance from the surface (I). Upon the tip reaching the near-surface (defined current decrease), the tip potential was stepped to VPulse = -400 mV for 50 ms (II) and then back to 50 mV as the tip was retracted from the surface (III). A similar potential pulse was applied in bulk solution (IV), before the probe was moved on to the next pixel (V) and the procedure was repeated. The current-time-tip position was measured synchronously throughout. 11 ACS Paragon Plus Environment


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Finite Element Method (FEM) simulations All FEM simulations were performed using COMSOL Multiphysics (5.3). Full details are presented in SI1, Section SI1-2 and a COMSOL report is given in SI2. Briefly, a 2D axisymmetric geometry representing the nanopipette probe positioned above a charged interface was constructed. The transport of diluted species, electrostatics and laminar flow modules were used to solve for the diffusion, electric field and electro-osmotic flow (EOF) problems simultaneously. Steady-state simulations were first performed at different probesubstrate distances in order to determine the separation distance which corresponded to the experimental set point, defined by the decrease in current compared to bulk (~30 nm herein, for a 3% decrease in tip current). Time (t)- dependent simulations to calculate the experimental current (I)-t response during the pulse were then performed at the calculated separation distance, and with the probe positioned in bulk solution, with different surface charges present on the substrate surface region (representing the hair surface) beneath the nanopipette. Given the large radius of curvature of hair (diameter ca. 50-100 m) compared to the tip diameter (220 nm), it was reasonable to consider a planar substrate surface. The calculations allowed for a working curve of normalized current (at a tip potential of -400 mV) versus surface charge to be generated. The normalized current was the value recorded at the near-surface compared to that in bulk solution, each at 50 ms after applying the potential of -400 mV (from a potential of 50 mV). These data were used to generate the surface charge maps presented herein. As outlined in SI1-2, a simple Gouy-Chapman model defined the double-layer at the surface and SICM tip, which was reasonable for the ionic strength used herein (50 mM KCl).33


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RESULTS AND DISCUSSION Effect of different treatments on the hair surface charge density Two different treatments were applied to hair that was cleaned and prepared as described in the Experimental Section. First, Figure 3 shows typical SICM surface charge density and corresponding topography maps of the same area collected before and after the application of bleach. Figure 3 (a) (i), shows that untreated hair has a fairly homogeneous surface charge density distribution, with the average across the scan being around -15 mC/m2. This negative charge at the pH of the solution of ca. 6.8, is qualitatively consistent with expectations from macroscopic zeta potential measurements.19, 20 The negative charge can be attributed to weak acid group functionalities within the lipid layer, which is partly permeable to water.13 SICM provides quantitative values for the surface charge and, furthermore, highlights the spatial distribution of surface charge. It can be seen that there are a few regions where the negative surface charge density is somewhat larger in magnitude, attaining values approaching values ca. -35 mC/m2. With respect to the corresponding topography image in Figure 3 (a) (ii), which shows an upper cuticle (left-hand side of the image over a lower cuticle on the right-hand side) these regions are localized and in regions of the lower cuticle platelet near the step to the upper cuticle. Figure 3 (b) (i) shows the same hair after exposure to a bleach treatment (see Experimental Section). There is evidently a dramatic change in the hair surface charge, with the appearance of much more negatively charged domains, where the negative surface charge density can attain values of -80 to -100 mC/m2. Hair bleaching oxidizes the amino acid cysteine within the protein matrix of the cortex, and other areas of the hair that are rich in cysteine inside the cuticle cells.48 The resulting outer surface becomes hydrophilic with SO3− (cysteic acid) end groups exposed, leading to a strong negative charge.11,

49

With reference to the 13

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corresponding topography map, Figure 3 (a) (ii), these regions are mainly around the edges of the cuticle platelets. As supplementary information, the normalized current data corresponding to the charge map data are shown in SI1, Section SI1-3, Figure S2. The working curve for converting normalized current values to surface charge density is shown in SI1, Section SI1-2, Figure S1. The second surface treatment explored was the use of a commercial hair conditioner. The main function of conditioners is to deposit surfactants onto the hair surface to give a smooth, shiny texture. Conditioning deposits a layer of cationic surfactants onto the hair which are attracted to the natural negative charge of the hair. It is thought that conditioning the hair helps seal the gaps that expose the cortex to more environmental damage.50 Figure 4 illustrates charge maps and the simultaneous topography maps of an untreated hair and after it had been conditioned. As in Figure 4 (a) (i), the untreated hair predominantly has a surface charge value of around -15 mC/m2. Upon treatment with the hair conditioner, a positive shift in the surface charge is observed over most of the surface, with regions with charge densities of 20 mC/m2 present. The charge on the surface is, however, heterogeneous, with some negatively charged domains still present, notably at the thin step edges between some of the platelets. Corresponding normalized current maps are presented in the SI1, Section SI-3, Figure S3. As described in the Experimental Section, the hair sample was thoroughly rinsed after the conditioning step, and so the data in Figure 4 indicate that, overall, the conditioning step produces a durable positive surface charge on the hair surface.


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Figure 3. (i) SICM surface charge and (ii) topography maps of a cleaned hair: (a) before and (b) after exposure to bleach. Note that there is no interpolation of any of the data in these maps. Each pixel is a discrete measurement.



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Figure 4. (i) SICM surface charge and (ii) topography maps of a cleaned hair: (a) before and (b) after conditioning. Note that there is no interpolation of any of the data in these maps. Each pixel is a discrete measurement.


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Effect of Bleach Followed by Conditioning on a Single Hair Fiber To further illustrate how SICM can be used to probe the effects of different surface treatments, SICM scans were performed on single hair fibers which had been treated with bleach and then conditioner. The images presented in this section (corresponding normalized current maps shown in the SI1, Section SI-4, Figure S4) are representative of studies of several different sample areas, with additional data for other regions of the hair samples, given in SI1, Section SI-4, Figure S5. Figure 5 (a) shows a typical charge map and simultaneous topography map of an untreated hair. Relatively homogeneous surface charge values are again observed across the hair surface as well as smooth surface topography of the platelets, with steps of up to 800 nm in height. This same hair was then exposed to the bleach treatment with the corresponding surface charge and topographical maps presented in Figure 5 (b). A similar trend to Figure 3 (a) is seen, with heterogeneous regions of highly negative surface charge (potential) due to the disruption to the outer protective layer and the disulfide bonds that are key to the stability of the hair keratin. Moreover, the surface topography becomes slightly more irregular with an increased surface roughness observed in the SICM topography map shown in Figure 5 (b) (ii). Figure 5 (c) illustrates the effect of conditioning the same hair that had been exposed to the bleach treatment. As in the case where the untreated hair was subjected to a conditioner treatment, there was an overall shift in the surface charge to positive values of around 5 mC/m2. Cationic surfactants present in the conditioner are evidently attracted to the negatively charged hair surface, reversing the overall net surface charge (Figure 5 (c) (i)). The charge maps seem to become more uniform across the surface of the hair upon conditioning. Chemically-treated (bleached) hair has a high affinity to conditioning ingredients as it has a very low isoelectric point, due to the formation of cysteic acid residues and is more porous (i.e. reduced cross-



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linking density) than untreated hair.4, 18, 51, 52 Moreover, surface roughness was reduced after conditioning the hair as shown in Figure 5 (c) (ii). Thus, the SICM studies highlights the role of conditioners which include

flattening of the cuticle cells, which makes the surface

smoother.4, 11, 53

Figure 5. (i) SICM charge maps and (ii) topography maps of the same hair fiber treated: (a) untreated, followed by (b) bleach and then (c) conditioner. The maps for each treatment were recorded in different areas. Note that there is no interpolation of any of the data in these maps. Each pixel is a discrete measurement.


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Surface charge maps at different positions on a single hair fiber As mentioned in the Experimental section, measurements were usually made on sections of hair close to the root. A further advantage of SICM is that we are readily able to image different parts of a single hair fiber and determine whether there are any differences in the surface charge distribution. To briefly illustrate this capability, two distinct regions of a single hair fiber of ~ 8 cm long were examined. Example surface charge maps are shown in Figure 6, with the corresponding normalized current values and topography maps presented in the SI1, Section SI-5, Figure S6. Closer to the root (~ 1 cm from the root), where the hair is fresher, the moderate negative surface charge is fairly homogeneous across the surface as illustrated in Figure 6 (a), with a corresponding smoother topography (SI1, Figure S6). However, further away from the root (~ 5 cm from the root), the surface charge distribution across the surface is more heterogeneous (Figure 6 (b). Regions of the hair that are further away from the root are more likely to have become damaged as these regions are older and have been exposed to the environment for longer.

Figure 6. Surface charge maps along the same hair with (a) showing a region close to the root of the hair and (b) showing a region near the end of the hair. Note that there is no interpolation of any of the data in these maps. Each pixel is a discrete measurement. 19 ACS Paragon Plus Environment


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CONCLUSIONS This work has demonstrated that SICM can be used to visualize and quantify the surface charge of human hair on the nanoscale, while also synchronously mapping the surface topography. We were able to observe differences in surface charge magnitude and distribution between native hair, and after chemical treatments including hair dyes (bleach) and conditioning products. These differences were rationalized based on chemical processes at the hair fiber surface. When treated with a bleach product, the natural lipid layer on the hair is disrupted, leading to greater surface charge heterogeneity, with regions of very high negative surface charge observed. The bleaching process breaks some of the disulfide bonds within the keratin and can damage the cuticle making it more porous, leading to increased surface roughness that was also seen in the SICM topographical maps. Conversely, conditioner products were observed to produce a positive surface charge, attributed to a layer of cationic surfactants on the surface of the hair. Surface charge is of ubiquitous interest in interfacial science, and we expect SICM to become a key technique with wide applications. There are relatively few techniques that are capable of quantitative surface charge-topography mapping. Powerful features of SICM have been illustrated in this paper, whereby new quantitative insights into the surface charge distribution of human hair have been revealed for the first time.

ACKNOWLEDGEMENTS FMM acknowledges Industrial Strategy Funding from the University of Warwick. DP acknowledges support from a Leverhulme Trust Research Project Grant. RB thanks EPSRC for a PhD studentship through the Centre for Doctoral Training in Molecular Analytical Science, grant EP/L015307/1. AP thanks EPSRC for a Ph.D. studentship through the MOAC 20 ACS Paragon Plus Environment

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DTC. PRU gratefully acknowledges support from a Royal Society Wolfson Research Merit Award.

SUPPORTING INFORMATION Supporting Information (SI) is freely available. SI1 contains further details on the composition (ingredients) of the treatments applied to hair, FEM simulations performed, normalized current maps that correspond to the surface charge data presented herein, and further exemplar data. SI2 is a COMSOL report for the simulations.



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REFERENCES 1.

Ruetsch, S. B.; Kamath, Y. K.; Kintrup, L.; Schwark, H. J., Effects of conditioners

on surface hardness of hair fibers: an investigation using atomic force microscopy. J. Cosmet. Sci. 2003, 54 (6), 579-88. 2.

Longo, V.; Monteiro, V.; Pinheiro, A.; Terci, D.; Vasconcelos, J.; Paskocimas, C.;

Leite, E.; Longo, E.; Varela, J. A., Charge density alterations in human hair fibers: an investigation using electrostatic force microscopy. Int. J. Cosmet. Sci. 2006, 28 (2), 95-101. 3.

Guthrie, J. T.; Kazlauciunas, A.; Rongong, L.; Rush, S., The characterisation of

treated and dyed hair. Dyes Pigm. 1995, 29 (1), 23-44. 4.

Gavazzoni Dias, M. F. R., Hair Cosmetics: An Overview. Int. J. Trichology 2015, 7

(1), 2-15. 5.

LaTorre, C.; Bhushan, B., Nanotribological characterization of human hair and skin

using atomic force microscopy. Ultramicroscopy 2005, 105 (1), 155-175. 6.

Yang, F.-C.; Zhang, Y.; Rheinstädter, M. C., The structure of people’s hair. PeerJ

2014, 2, e619. 7.

Wei, G.; Bhushan, B.; Torgerson, P. M., Nanomechanical characterization of human

hair using nanoindentation and SEM. Ultramicroscopy 2005, 105 (1), 248-266. 8.

Bhushan, B.; Chen, N., AFM studies of environmental effects on nanomechanical

properties and cellular structure of human hair. Ultramicroscopy 2006, 106 (8), 755-764. 9.

McMullen, R. L.; Kelty, S. P., Investigation of human hair fibers using lateral force

microscopy. Scanning 2001, 23 (5), 337-345. 10.

LaTorre, C.; Bhushan, B., Investigation of scale effects and directionality dependence

on friction and adhesion of human hair using AFM and macroscale friction test apparatus. Ultramicroscopy 2006, 106 (8), 720-734.


Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11.

Robbins, C. R., Chemical and physical behavior of human hair. Springer: 2002; Vol.

4. 12.

Bhushan, B., Biophysics of human hair: structural, nanomechanical, and

nanotribological studies. Springer Science & Business Media: 2010. 13.

Cheong, D. W.; Lim, F. C. H.; Zhang, L., Insights into the Structure of Covalently

Bound Fatty Acid Monolayers on a Simplified Model of the Hair Epicuticle from Molecular Dynamics Simulations. Langmuir 2012, 28 (36), 13008-13017. 14.

Wolfram, L. J., Human hair: A unique physicochemical composite. J. Am. Acad.

Dermatol. 2003, 48 (6, Supplement), S106-S114. 15.

Aguh, C.; Okoye, G., Fundamentals of Ethnic Hair: The Dermatologist's Perspective.

Springer: 2016. 16.

Dias, M. F. R. G., Hair cosmetics: an overview. Int. J. Trichology 2015, 7 (1), 2.

17.

Wolfram, L. J., Human hair: a unique physicochemical composite. J. Am. Acad.

Dermatol. 2003, 48 (6), S106-S114. 18.

Wilkerson, V. A., The chemistry of human epidermis ii. the isoelectric points of the

stratum corneum, hair, and nails as determined by electrophoresis. J. Bio. Chem. 1935, 112 (1), 329-335. 19.

Jachowicz, J.; Berthiaume, M.; Garcia, M., The effect of the amphiprotic nature of

human hair keratin on the adsorption of high charge density cationic polyelectrolytes. Colloid Polym. Sci. 1985, 263 (10), 847-858. 20.

Tagliaro, F.; Smith, F.; De Battisti, Z.; Manetto, G.; Marigo, M., Hair analysis, a

novel tool in forensic and biomedical sciences: new chromatographic and electrophoretic/electrokinetic analytical strategies. J. Chromatogr. B Biomed. Sci. Appl. 1997, 689 (1), 261-271.



21.

Page 24 of 27

Jacobasch, H.-J.; Bauböck, G.; Schurz, J., Problems and results of zeta-potential

measurements on fibers. Colloid Polym. Sci. 1985, 263 (1), 3-24. 22.

Buk, H., Zeta potential determination of polymeric materials using two differently

designed measuring cells of an electrokinetic analyser. Acta Chim. Slov. 2010, 57, 700-706. 23.

Lodge, R. A.; Bhushan, B., Surface potential measurement of human hair using

Kelvin probe microscopy. J. Vac. Sci. Technol. A 2007, 25 (4), 893-902. 24.

Bhattacharjee, S., DLS and zeta potential – What they are and what they are not? J.

Control. Release 2016, 235, 337-351. 25.

Longo, V. M.; Monteiro, V. F.; Pinheiro, A. S.; Terci, D.; Vasconcelos, J. S.;

Paskocimas, C. A.; Leite, E. R.; Longo, E.; Varela, J. A., Charge density alterations in human hair fibers: an investigation using electrostatic force microscopy. International J. Cosmet. Sci. 2006, 28 (2), 95-101. 26.

Dupres, V.; Langevin, D.; Guenoun, P.; Checco, A.; Luengo, G.; Leroy, F.,

Wetting and electrical properties of the human hair surface: Delipidation observed at the nanoscale. J. Colloid Int. Sci. 2007, 306 (1), 34-40. 27.

Lodge, R. A.; Bhushan, B. J., Surface potential measurement of human hair using

Kelvin probe microscopy. J. Vac. Sci. Technol. A 2007, 25 (4), 893-902. 28.

Stone, A. D. D.; Mesquida, P., Kelvin-probe force microscopy of the pH-dependent

charge of functional groups. Appl. Phys. Lett. 2016, 108 (23), 233702. 29.

Gouveia, R. F.; Galembeck, Electrostatic charging of hydrophilic particles due to

water adsorption. J. Am. Chem. Soc. 2009, 131 (32), 11381-11386. 30.

Salerno, M.; Dante, S., Scanning Kelvin Probe Microscopy: Challenges and

Perspectives towards Increased Application on Biomaterials and Biological Samples. Materials 2018, 11 (6), 951.


Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


31.

Chen, C.-C.; Zhou, Y.; Baker, L. A., Scanning ion conductance microscopy. Annu.

Rev. Anal. Chem. 2012, 5, 207-228. 32.

Chen, C.-C.; Baker, L. A. J. A., Effects of pipette modulation and imaging distances

on ion currents measured with scanning ion conductance microscopy (SICM). Analyst 2011, 136 (1), 90-97. 33.

Page, A.; Perry, D.; Unwin, P. R., Multifunctional scanning ion conductance

microscopy. Proc. Math. Phys. Eng. Sci. 2017, 473 (2200), 20160889-20160889. 34.

Rheinlaender, J.; Geisse, N. A.; Proksch, R.; Schäffer, T. E. J. L., Comparison of

scanning ion conductance microscopy with atomic force microscopy for cell imaging. Langmuir 2010, 27 (2), 697-704. 35.

Sa, N.; Lan, W.-J.; Shi, W.; Baker, L. A., Rectification of ion current in nanopipettes

by external substrates. ACS nano 2013, 7 (12), 11272-11282. 36.

Clarke, R. W.; Zhukov, A.; Richards, O.; Johnson, N.; Ostanin, V.; Klenerman, D.,

Pipette–surface interaction: current enhancement and intrinsic force. J. Am. Chem. Soc. 2012, 135 (1), 322-329. 37.

Perry, D.; Paulose Nadappuram, B.; Momotenko, D.; Voyias, P. D.; Page, A.;

Tripathi, G.; Frenguelli, B. G.; Unwin, P. R., Surface Charge Visualization at Viable Living Cells. J. Am. Chem. Soc. 2016, 138 (9), 3152-3160. 38.

McKelvey, K.; Kinnear, S. L.; Perry, D.; Momotenko, D.; Unwin, P. R., Surface

Charge Mapping with a Nanopipette. J. Am. Chem. Soc. 2014, 136 (39), 13735-13744. 39.

Perry, D.; Al Botros, R.; Momotenko, D.; Kinnear, S. L.; Unwin, P. R.,

Simultaneous Nanoscale Surface Charge and Topographical Mapping. ACS Nano 2015, 9 (7), 7266-7276.



40.

Page 26 of 27

Page, A.; Perry, D.; Young, P.; Mitchell, D.; Frenguelli, B. G.; Unwin, P. R., Fast

Nanoscale Surface Charge Mapping with Pulsed-Potential Scanning Ion Conductance Microscopy. Anal. Chem. 2016, 88 (22), 10854-10859. 41.

Dorwling-Carter, L.; Aramesh, M.; Han, H.; Zambelli, T.; Momotenko, D.,

Combined Ion Conductance and Atomic Force Microscope for Fast Simultaneous Topographical and Surface Charge Imaging. Anal. Chem. 2018, 90 (19), 11453-11460. 42.

Momotenko, D.; McKelvey, K.; Kang, M.; Meloni, G. N.; Unwin, P. R.,

Simultaneous interfacial reactivity and topography mapping with scanning ion conductance microscopy. Anal. Chem. 2016, 88 (5), 2838-2846. 43.

Zhu, C.; Zhou, L.; Choi, M.; Baker, L. A., Mapping Surface Charge of Individual

Microdomains with Scanning Ion Conductance Microscopy. ChemElectroChem 2018, 5 (20), 2986-2990. 44.

Kang, M.; Perry, D.; Bentley, C. L.; West, G.; Page, A.; Unwin, P. R.,

Simultaneous topography and reaction flux mapping at and around electrocatalytic nanoparticles. ACS Nano 2017, 11 (9), 9525-9535. 45.

Takahashi, Y.; Murakami, Y.; Nagamine, K.; Shiku, H.; Aoyagi, S.; Yasukawa, T.;

Kanzaki, M.; Matsue, T., Topographic imaging of convoluted surface of live cells by scanning ion conductance microscopy in a standing approach mode. Phys. Chem. Chem. Phys. 2010, 12 (34), 10012-10017. 46.

Novak, P.; Li, C.; Shevchuk, A. I.; Stepanyan, R.; Caldwell, M.; Hughes, S.;

Smart, T. G.; Gorelik, J.; Ostanin, V. P.; Lab, M. J.; Moss, G. W.; Frolenkov, G. I.; Klenerman, D.; Korchev, Y. E., Nanoscale live-cell imaging using hopping probe ion conductance microscopy. Nat. Methods 2009, 6 (4), 279.


Page 27 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


47.

Bossers, L. C. A. M.; Paul, R.; Berry, A. J.; Kingston, R.; Middendorp, C.; Guwy,

A. J., An evaluation of washing and extraction techniques in the analysis of ethyl glucuronide and fatty acid ethyl esters from hair samples. J. Chromatogr. B 2014, 953-954, 115-119. 48.

Robbins, C. R., Bleaching and Oxidation of Human Hair. In Chemical and Physical

Behavior of Human Hair, Springer Berlin Heidelberg: Berlin, Heidelberg, 2012; pp 263-328. 49.

Korte, M.; Akari, S.; Kühn, H.; Baghdadli, N.; Möhwald, H.; Luengo, G. S.,

Distribution and Localization of Hydrophobic and Ionic Chemical Groups at the Surface of Bleached Human Hair Fibers. Langmuir 2014, 30 (41), 12124-12129. 50.

Cruz, C.; Costa, C.; Gomes, A.; Matamá, T.; Cavaco-Paulo, A., Human Hair and the

Impact of Cosmetic Procedures: A Review on Cleansing and Shape-Modulating Cosmetics. Cosmetics 2016, 3 (3), 26. 51.

Cruz, C. F.; Martins, M.; Egipto, J.; Osorio, H.; Ribeiro, A.; Cavaco-Paulo, A.,

Changing the shape of hair with keratin peptides. RSC Advances 2017, 7 (81), 51581-51592. 52.

Wolfram, L. J.; Hall, K.; Hui, I., The mechanism of hair bleaching. J. Soc. Cosmet.

Chem 1970, 21, 875-900. 53.

Dawber, R., Hair: Its structure and response to cosmetic preparations. Clin. Dermatol.

1996, 14 (1), 105-112.


Nanoscale Surface Charge Visualization of Human Hair - Analytical

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