Coloration of colloidal polymer particles through selective extraction of Mie backscattering for cation-responsible colorimetric sensors

Journal of Colloid and Interface Science xxx (xxxx) xxx

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Coloration of colloidal polymer particles through selective extraction of Mie backscattering for cation-responsible colorimetric sensors Seung-Jea Lee a, Santosh Kumar a, Jin Woo Choi a,b, Jae-Suk Lee a,⇑ a School of Materials Science and Engineering, Grubbs Center for Polymers and Catalysis, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro Buk-gu, Gwangju 61005, Republic of Korea b Surface Technology Division, Korea Institute of Materials Science (KIMS), Changwon 51508, Republic of Korea

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 25 July 2019 Revised 16 October 2019 Accepted 18 October 2019 Available online xxxx Keywords: Mie scattering Structural colour Polymer particle

a b s t r a c t Mie backscattering-based coloration of water-dispersed polymer particles is investigated. Diluted colloids and light absorber-added colloids are individually prepared to study the scattering properties. Light absorber-added colloids show ~2.9-fold higher colour saturation than diluted colloids because of the effective elimination of Mie forward scattering. Mechanism of selective extraction of Mie backscattering-based colour in the presence of light absorbers is explained by path length difference of Mie forward and backscattering. Using stimuli-responsive polymer particles, Mie scattering colourbased sensors that respond to pH and heavy metal ions are prepared. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Spherical particles have been used as structural colour materials that express colour through the scattering of light [1–3]. The characteristics of light scattered by spherical particles are determined by structure factor, relating to particle-particle distance, and form factor, relating to intrinsic properties of the particles like size and refractive index. Depending on the phase of the

⇑ Corresponding author.

distributed particles, the primary factor that determines the scattering characteristic of the colloid is defined [4,5]. If the particles are assembled in a crystal phase, the structure factor-dominated scattering occurs, and in the glass phase, structure factor and form factor simultaneously affect the scattering. If the colloid is in the gas phase, form factor determines the scattering properties. Structure factor induces coherent scattering which can be explained by Bragg’s law [6]. Finely-ordered assembly of spherical particles, so-called 3-dimensional photonic crystals, exhibit reflective colours that change depending on the inter-particle distance [7–10]. Distance-dependent colour change expanded the usage of

E-mail address: [email protected] (J.-S. Lee). https://doi.org/10.1016/j.jcis.2019.10.073 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

Please cite this article as: S.-J. Lee, S. Kumar, J. W. Choi et al., Coloration of colloidal polymer particles through selective extraction of Mie backscattering for cation-responsible colorimetric sensors, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.073

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the photonic crystals affording active colour materials for diverse applications, such as displays and sensors [11–13]. However, iridescence and severe angle-dependency of the colours realized by photonic crystals are problems that need to be solved [14,15]. Form factor induces incoherent scattering which has been interpreted by Mie theory [16,17]. The incoherently scattered light consists of strong forward scattering and weak backscattering. For submicron-sized low refractive index particles (refractive index < 2), the forward scattering scatters entire visible light, whereas backscattered light diffusively reflects a specific colour according to the size of the scatterers. However, multiple scattering of intense forward scattering disturbs the revelation of backscattering-based colours. To realize Mie backscattering-based coloration, it is required to eliminate multiple scattering of light. Retsch et al. used hollow silica particles to increase the transport mean free path (l*) of the scattered light. l* denotes the length that light propagates without changing direction [18]. Thus, as l* increases, the number of scattering events was reduced, and the colours of Mie backscattering was displayed. However, a low refractive index of the hollow silica particles caused weak backscattering, which resulted in low colour saturation. To enhance the saturation, Kim et al. coated carbon black in the hollow silica particles [19]. Carbon black-coated hollow silica particles exhibited colours with increased saturation at the concentrated phase, but the saturation was still low in the diluted colloidal state because of the low refractive index of the particles. Cho et al. suggested another approach using lightabsorbing particles [20]. They synthesized polydopamine particles that absorb blue light thus inhibiting multiple scattering. However, due to the strong absorption characteristics of polydopamine, an intense light source was required to observe Mie backscatteringbased coloration. So far, particles having a low effective refractive index (neff, < 1.1) or having light absorption properties (extinction coefficient (j) – 0) were used to achieve Mie backscattering-based coloration. Thus, the displayed colours had low colour saturation or demanded intense illumination. In addition, the particles were too rigid to be used as active colour materials. Non-absorbing (j = 0) polymer particles, which have a higher refractive index (1.1 < neff < 1.6) than hollow silica particles, are possible alternatives for Mie backscattering-based coloration to attain colours with increased saturation and functionality. Because of the higher refractive index, Mie backscattering efficiency of polymer particles is higher than the hollow silica particles. Thus, it is expected that the Mie backscattering-based colours of polymer particles should have higher colour saturation than the silica particles. However, in actuality, Mie scattering-based colours of polymer particle dispersions have low colour saturation (~0%, white) because the enhanced refractive index increases not only the efficiency of Mie backscattering but also the efficiency of Mie forward scattering which drops the colour saturation through multiple scattering. Therefore, in order to carry out Mie backscattering-based colours from polymer particles, selective removal of Mie forward scattering is needed. Herein, we used non-absorbing polymer particles to realize Mie backscattering-based colour with increased colour saturation. Mie scattering of polymer particles was characterized using polystyrene colloidal particles. To suppress the multiple scattering, increasing l* by diluting the colloids or dispersing carbon black to the colloidal solution were individually conducted. The influence of the carbon black on hue and saturation of the Mie backscattering-based colour was investigated. In addition, we prepared stimuli responsible polymer particles with 4-vinylpyridine to investigate the tunability of the Mie backscattering-based colour. The size changes of the polymer particles according to pH changes and the addition of heavy metal ions were monitored to

demonstrate that the Mie backscattering-based colours of the stimuli-responsible polymer particles could be tuned. 2. Materials and methods 2.1. Materials Styrene (99.9%, St), 4-vinylpyridine (95%, 4VP), Nisopropylacrylamide (97%, NIPAM), N, N0 -methylenebisacrylamide (99%, MBAAm) were purchased from Sigma-Aldrich. St and 4VP were passed through an aluminum oxide column before use. Bis[2-(4,5-dihydro-1H-imidazol-2-yl)propan-2-yl]diazene;dihydrochloride (98%, VA-044, Wako Chemical), Brij 98Ò (SigmaAldrich), cadmium dichloride (99.999%, CdCl2, Sigma-Aldrich) and mercury dichloride (99.5%, HgCl2, Alfa Aesar) were used without further purification. Hydrogen chloride (37%, HCl), ammonium hydroxide (28%, NH4OH) were purchased from Sigma-Aldrich and diluted to 1 M before use. Deionized (DI) water (14–18 MX cm), carbon black (CB) (99.9%, bulk density 170–230 g/L, Alfa Aesar) were used without further purification. 2.2. Preparation of poly(St), poly(4VP) particles Water-dispersed polymer particles were prepared following our previous method [21,22]. In a typical experiment for poly(St) (PS), 40 mL DI water and 3.2 mL (27.9 mmol) of St was added to a round-bottom flask. The content of the flask was bubbled with argon gas for 30 min. The solution was placed in an 80 °C-oil bath. Then, an aqueous solution of VA-044 (molar ratio of VA-044/ monomer = 0.01), purged with argon, was injected. The reaction was complete after 3 h to produce PS364. PS310 and PS270 were prepared by adjusting St amounts: 2.7 mL (23.6 mmol) for PS310, 2.2 mL (19.2 mmol) for PS270, respectively. For the preparation of poly(4VP) (P4VP) particles, 0.5 mL (4.6 mmol) of 4VP, 0.5 g (4.4 mmol) of NIPAM, and 0.1 g (0.6 mmol) MBAAm was added to a round-bottom flask containing 40 mL DI water. After 30 min of deoxygenation with argon gas, the flask placed in a 70 °C-oil bath. 0.28 mmol of VA-044 was added to the solution to start the reaction. The reaction was complete after 2 h. The concentration of the colloids was adjusted by additional DI water. 2.3. Preparation of CB dispersion 50 mg of CB and 5 mg of BrijÒ 98 was added to 10 mL of DI water. The solution was ultra-sonicated for 40 min. 2.4. pH adjustment of P4VP dispersion and heavy metal ion addition The pH of P4VP dispersion was adjusted by 1 M HCl and 1 M NH4OH. 0.1 g of CdCl2, and HgCl2 were dissolved in 10 mL of DI water, respectively. 0.55 mL of the CdCl2 solution and 0.81 mL of the HgCl2 solution was added to 3 mL of 0.1% P4VP to adjust the molar concentration to 10 mM. 2.5. Simulations Mie scattering of PS particles was simulated by the Mie scattering calculator kindly provided by Prof. Scott Prahl on an openaccess website (https://omlc.org). 2.6. Characterization An electrophoretic light scattering spectrophotometer (ELS-Z2 Otsuka Electronics Co., Japan) was used to analyze the hydrody-

Please cite this article as: S.-J. Lee, S. Kumar, J. W. Choi et al., Coloration of colloidal polymer particles through selective extraction of Mie backscattering for cation-responsible colorimetric sensors, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.073

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namic diameter of the polymer particles. The measurement was repeated 70 times to estimate the diameters. The diffuse reflectance of colloids was measured by a UV/Vis/NIR spectrophotometer (Cary 5000, Agilent) equipped with a diffuse reflectance accessory (Internal DRA-900) and a PMMA cuvette. The diffuse reflectance was scanned from 800 nm to 380 nm by a quartz iodine lamp as a light source. The light source is on the opposite side of the sample port with the integral sphere in between. The diffuse reflection was integrated with the sphere and probed by a photomultiplier tube (PMT) detector at the top of the sphere. Theoretical colours of samples, which have two diffusive reflection peaks within the visible spectrum, were estimated by an online colour mixing tool (https://trycolors.com). Photographs of the colloids were obtained by a digital camera. A homemade mini-darkroom was used for taking clear images. The darkroom was 1.5 cm wide, 4.5 cm high, and 8.5 cm long. One side of the darkroom was opened for placing camera lens and flashlight. Glass vial with colloidal solutions was placed at the closed side, and the camera was placed at the open side. The distance between the sample and the flashlight was 7.0 cm. The camera was sloped ~5° so that the camera and the glass vial may not be parallel. The luminous flux of the flashlight was 26 lm. Colours of the photographs were converted to red-greenblue (RGB) values, hue, saturation, and brightness by the ‘Eyedropper’ function of PowerPoint2016 (Microsoft).

3. Results and discussion 3.1. Mie backscattering-based colours of diluted PS dispersions Mie scattering of non-absorbing polymer particles was characterized using water-dispersed PS particles with 364 nm (PS364), 310 nm (PS310), and 270 nm (PS270) in diameter. Polydispersity index (PDI) of these particles were 0.017, 0.065, and 0.030 for PS364, PS310, and PS270, respectively. Regardless of the particle size, the peak value of total scattering efficiency (Qsca) was at a shorter wavelength and gradually decreased toward longer wavelengths (Fig. 1a). The total scattering efficiency is defined as Qsca = rsca pr2 , where rsca is the scattering cross-section; the ratio of the inten-

sity of radiant energy scattered in all directions to the incident irraR 2p diance (rsca ¼ 0 ddhr dh) and r is the radius of the scatterers. Mie scattering is composed of intense forward scattering and weak backscattering. Thus, Qsca of Mie scattering to some extent represents Mie forward scattering. On the contrary, peaks of Mie backscattering efficiency (Qbsc) changed according to particle size , (Fig. 1b). Mie backscattering efficiency is defined as Qbsc= rpbsc r2 where rbsc is the backscattering cross-section; the ratio of the intensity of radiant energy scattered into the back in the direction R 3p of the incident irradiance (rbsc ¼ p2 ddhr dh). Although Mie backscat2

tering induces diffusive reflection of specific wavelengths that may exhibit colours, colloids usually are white due to the multiple scattering of intense Mie forward scattering. The number of scattering is related to the l*. The longer the l*, the fewer the number of scattering events occurs. l* is represented by the following equation [23]:

l ¼ 1=ðqrð1  gÞÞ 

ð1Þ

where g is an anisotropy parameter, q is the number density of scatterers dispersed in media, r is the scattering cross-section. l* of PS364 was calculated using Eq. (1) (Fig. 1c). The number density of the particles was substituted to mass density utilizing the density of the PS particles (1.05 g/mL) and the volume of an individual particle (2.53  1014 mL). Decreasing the concentration of PS particles results in an exponential increase of l*. Thus, diluting the colloids would reduce the number of scattering and possibly exhibit Mie backscattering-based colours. Fig. 1d shows particle concentration-dependent colours of PS particle dispersions. The colour of PS364 changed from white to greenish-grey as the concentration decreased, whereas PS310 and PS270 showed blue and magenta colours, respectively. Colours of the colloids were related to the size of the dispersed particles. Hue, saturation, and brightness of the colours are indicated in Table S1. The backscattered light of 0.01% PS364, PS310, and PS270 was characterized by diffuse reflectance spectroscopy which probes incoherent reflections (Fig. 1e). Peaks were at 388 nm, and 531 nm for PS364, 454 nm for PS310, and 399 nm, and 778 nm for

Fig. 1. Simulation of Mie scattering of PS364 (solid line), PS310 (dash line), and PS270 (dot line): (a) scattering efficiency (Qsca), (b) backscattering efficiency (Qbsc). (c) Transport mean free path (l*) with respect to the particle concentration of PS364 at 565 nm. (d) Photographs of PS364, PS310, and PS270 in different concentrations. (e) Diffuse reflectance of PS364, PS310, and PS270. Solid lines represent Gaussian fitting.

Please cite this article as: S.-J. Lee, S. Kumar, J. W. Choi et al., Coloration of colloidal polymer particles through selective extraction of Mie backscattering for cation-responsible colorimetric sensors, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.073

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PS270. Using colour mixing tool, colours corresponding to each peak were combined to estimate the theoretical colours (Figure S1). The hue of the estimated colours matched with those of the experimentally measured colours. This indicates that the peaks existing in the visible spectrum contribute to the colour exhibition. Furthermore, the measured wavelength of the peak (kpeaks) corresponded with the calculated kpeaks in Fig. 1b (Table S2). Thus, it was confirmed that the colours of dilute colloids are Mie backscattering-based colours. Magkiriadou et al. suggested that in the colloidal system of spherical polymer particles, Mie scattering and Fabry-Perot resonance independently affect kpeak of backscattered light [24]. Thus, we calculated Fabry-Perot resonance-induced kpeak of PS particles for a better understanding of the colours of dilute colloids. FabryPerot resonance can be calculated by the following equation:

kpeak ¼ ð2np DÞ=z

ð2Þ

narrow because the composition of Mie forward scattering in the diffuse reflection was reduced (Figure S4). This indicates that dispersing CB more efficiently removes Mie forward scattering than increasing l* by diluting the colloids. The addition of CB also changed the hue of colours. For PS364 and PS270, hue of the colours changed from 69° to 98° and 323° to 287°, respectively. For PS310, the change was negligible (229– 222°). The shift of the hue indicates that mode2 peaks became colour-determining peaks after the addition of CB. This is because the absorption of CB in the solution caused the intensity of mode2 peaks to become relatively higher than the mode1 and mode3 peaks. 3.3. CB-assisted selective extraction of Mie backscattering-based colours

where np is the refractive index of the particle (1.59 for PS particles), D is the diameter of the particle, and z is an integer that designates the mode of the resonance. Solid lines in Figure S2 indicate particle diameter-dependent kpeak when z = 1, 2, 3. Interestingly, kpeak of Fabry-Perot resonance calculated by Eq. (2) coincides with kpeak estimated by Mie theory. This indicates that Fabry-Perot resonance and Mie backscattering of a spherical polymer particle are related. The calculated Mie backscattering peaks at 860 nm of PS270 in Fig. 1b overlapped on the mode1 line of Fabry-Perot resonance calculated by Eq. (2) (Figure S2, z = 1). Likewise, the second (399 nm for PS270, 462 nm for PS310, and 542 nm for PS364) and the third Mie backscattering peaks (at 328 nm for PS310, and 385 nm for PS364) in Fig. 1b were respectively overlapped on the mode2 (z = 2) and mode3 (z = 3) lines in Figure S2. For this reason, hereafter the first Mie backscattering peaks, counting from the longer wavelength, were categorized as mode1 and the second and the third peaks as mode2 and mode3, respectively.

When CB added the influence of intense Mie forward scattering on the colours of colloids became negligible even though Mie forward scattering is 55.6 times intenser than Mie backscattering (for PS364 at kpeak of mode2, Figure S5). Mechanism of selective absorption of Mie forward scattering was investigated geometrically. We assumed that the scattered light propagates in a direction of 0° or 180° with regard to the incident light, and light scatters elastically. The propagation of the Mie scattered light can be described as shown in the scheme in Fig. 2d. The colours of colloids are observed from the opposite direction of the incident light (180°). To reverse the direction of the propagation, at least two times of elastic scattering is required. In other words, the scattered light reverses its direction after traversing at least 3l*. Dispersed CB absorbs the propagating photons resulting in the reduction of the intensity of light. The relative intensity of light (If/Io) after a certain number of scattering events was calculated. Assuming that Io is unity If/Io is equal to the transmittance (T) that can be calculated with the absorbance of the CB dispersion. Using Beer-Lambert law,

3.2. Mie scattering-based colours of CB-added colloids

T ¼ 10A ¼ 10ðecLÞ

Colour saturation of the dilute colloids (0.01%) were 5.3%, 17.7%, and 10.8% for PS364, PS310, and PS270, respectively (Table S1). To increase the colour saturation, increasing the number of scatterers is required. However, this reduces l*, which causes multiple scattering of Mie forward scattering and whitening of the colloids. Selective removal of light scattered by Mie forward scattering in a short l* colloidal system was attempted by adding lightabsorbing materials. CB dispersed solution was added to the 0.1% colloids. As the concentration of CB increased, the colours of the colloids changed from white to dark green, blue, and purple for PS364, PS310, and PS270, respectively (Fig. 2a). Saturation of the colours was increased as the concentration of CB increased but began to saturate at around 0.08 g/L (Fig. 2b, Table S3). Thus, colloids containing 0.08 g/L of CB were selected for further investigation. Saturation of the colours was 12.8%, 28.8%, and 17.8% for PS364, PS310, and PS270, which was 7.5%, 11.1%, and 7.0% increase compared to 0.01% colloids, respectively. However, the colour became dark as the mass ratio of CB/polymer particles exceeds 0.3 (Figure S3). This indicates that at a moderate concentration of CB, Mie forward scattering is selectively absorbed, but at a higher concentration, Mie backscattering is absorbed as well. Fig. 2c shows that kpeak of CB added colloids nearly matches with the peaks in Fig. 1e. kpeak of mode2 was at 542 nm, 462 nm, and 390 nm for PS364, PS310, and PS270, respectively. Thus, it is reasonable to accept that the added CB does not hinder the Mie backscattering of the system. The full width at half maximum (FWHM) of the kpeak of 0.1% PS364 with CB was ~115 nm, which was ~49 nm narrower than the FWHM of 0.01% colloid without CB (FWHM = ~164 nm). FWHM of the backscattered light became

ð3Þ

where A is the absorbance of CB dispersion, e is the mass attenuation coefficient, c is the mass concentration of CB (24.9 g/mL, 12.5 g/mL), L is the length of the measured sample (1.1 cm). e was calculated by A = ecL (Figure S6). To estimate T after N times of scattering, L in Eq. (3) was substituted by (N + 1) l*. The calculated value shows that after 24 times scattering, the intensity of Mie forward scattering becomes weaker (0.017) than the Mie backscattering (0.018) (Fig. 2d). Since light propagates randomly, N is higher than 24 in a real situation. Therefore, when CB is present, the diffuse reflectance of the colloids mainly consists of Mie backscattering because Mie forward scattering loses its intensity during longer propagation. Distance between surface and scattering sites affects the extinction of backscattered light. As Fig. 2e shows, the extinction of backscattered light proportionally increases as the length of propagation increased. Fig. 2e is a simplified simulation that only considers the absorption of CB, but in reality, the backscattered light would also be extinct by other factors. Thus, the probability of observing backscattered light of particles existing distant from the surface is very low. On the contrary, Mie backscattering near the surface may not be hindered by the absorption of CB nor secondary scattering due to other particles. Thus, particles existing near the surface participate in realizing Mie backscattering-based colours when CB is added to the system. 3.4. Colour saturation depending on the colloidal concentration To investigate the correlation between the concentration of colloids and colour saturation, the concentration of the colloids was adjusted to 4.0% (Fig. 3a). Despite the increased concentration of

Please cite this article as: S.-J. Lee, S. Kumar, J. W. Choi et al., Coloration of colloidal polymer particles through selective extraction of Mie backscattering for cation-responsible colorimetric sensors, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.073

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Fig. 2. (a) Photographs and RGB-converted images of 0.1% PS364, PS310, and PS270 with respect to carbon black (CB) concentrations (g/L). (b) Saturation (%) with respect to CB concentration of PS364 (square), PS310 (circle), and PS270 (triangle). Solid lines represent nonlinear fitting. (c) The diffuse reflectance of 0.08 g/L CB added PS364, PS310, and PS270. Solid lines represent Gaussian fitting. (d) Schematic image describing the propagation of the scattered light (left) and the calculated transmittance of 0.08 g/L CB dispersion with respect to the number of scattering. (e) Schematic image describing extinction of backscattered light (left) and calculated absorbance of 0.08 g/L CB dispersion with respect to distance.

Fig. 3. (a) Photographs and RGB-converted images of 4.0% SV380, SV353, and SV269 with respect to carbon black (CB) concentrations (g/L). (b) RGB-converted images of 0.1% PS270 and 4.0% SV269 with respect to the mass ratio of CB/polymer particles (CB/PP). (c) Saturation (%) of 0.1% PS270 (square) and 4.0% SV269 (circle) with respect to CB/PP (%).

the particles, which tremendously decreases l*, Mie backscattering-based colour was revealed when CB was added. This supports our interpretation that the coloration was conducted

by selective absorption of Mie forward scattering, irrespective of l*. A higher concentration of CB was needed for 4.0% colloid than 0.1% in order to reveal Mie backscattering-based colour (Table S4).

Please cite this article as: S.-J. Lee, S. Kumar, J. W. Choi et al., Coloration of colloidal polymer particles through selective extraction of Mie backscattering for cation-responsible colorimetric sensors, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.073

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However, the relative amount of CB to polymer particles (CB/PP) was comparable. The mass ratio of CB/PP is indicated in Table S3 and Table S4. Samples with 4.0% and 0.1% colloids showed that colours begin to saturate at CB/PP = 0.08 (8.0%). Interestingly, the colour saturation of 4.0% was higher than 0.1% at similar CB/PP ratio (Fig. 3b, c). This is because of an increased number of scatterers at the surface of the system that produces colour through Mie backscattering. In addition, 4.0% colloids showed discernible colours regardless of the viewing angles even under weak light due to the increased number of scatterers (Figure S7). 3.5. Particle size-dependency of Mie backscattering-based colours Water-dispersed polymer particles of various sizes were prepared to inspect the size-dependency of Mie backscatteringbased colour. The particle size was controlled by St/4VP ratio as described in our previous works (Table S5).[21,22] The particle size linearly increased as St/4VP ratio decreased (Fig. 4a). Fig. 4b shows that kpeak of the diffuse reflection shifted depending on the particle size. For SV166, SV199, SV204, and SV230, mode1 of Mie backscattering determined the colour of the colloids. As indicated in Fig. 4c, SV166 showed blue colour because the broad peak of mode1 existed at short wavelength. As particle size gets larger, peak of mode1 redshifted. SV199 and SV204 showed grey colours even though kpeak of them were at 519 nm and 534 nm, respectively, because the peaks were so broad that they diffusively reflected the whole range of the visible spectrum. The mode1 peak of SV230 was also broad, but it showed gold-like colour because the peak did not cover ~400 nm. From SV269, peaks of mode2 defined the colour. SV269, SV296, SV353, and SV380 had their kpeak of mode2 at 377 nm, 413 nm, and 476 nm, and 520 nm, respectively. The theoretical colour corresponding to the kpeaks was obtained by converting wavelengths to hue and adjusting saturation and brightness to 20% and 39%, respectively (Fig. 4d). The difference

between theoretical hue and measured hue was less than 26° which is 29 nm when it was converted to wavelength. This indicates that peaks of mode2 primarily influenced Mie backscattering-based coloration over weak peaks of mode1 and mode3 in the visible range. SV424 had peaks of mode3 and mode2 at 398 nm and 588 nm, respectively. Since the intensity of the two peaks was comparable, the peaks simultaneously affected the colour (Fig. 4e). Using the size-dependency of Mie scattering-based coloration, polymer particles-based stimuli-responsible colour material was prepared. Poly(4-vinylpyridne) (P4VP) swells in acidic conditions due to the protonation of pyridine groups as schematically shown in Fig. 5a [25,26]. In acidic condition, the diameter of P4VP particles grew up to ~623 nm, and increased nearly 200 nm, whereas, in alkaline condition, diameter change was unremarkable, ~8 nm (Fig. 5b). The kpeak of backscattered light also changed correspondingly to the particle size (Fig. 5c). The measured kpeak at acidic condition was shorter than the calculated values because the refractive index of the particles was reduced due to penetrated water (Figure S8) [27]. At pH3 and pH4, peaks of mode2 and mode3 coexisted in the visible spectrum (Fig. 5d). Therefore, the colours of the colloids at pH3, pH4 were determined by the two peaks (Figure S9). The hue of colours changed drastically at acidic conditions, and colours were distinguishable. However, at alkali conditions, the tone of colours did not change visibly as well as the particle size (Fig. 5e). Furthermore, changes of Mie backscattering-based colour of the P4VP particles in accordance with the addition of heavy metal ions was investigated. The water solution of CdCl2 and HgCl2 were individually added to P4VP dispersions of pH3. When the metal ions were added, particle size decreased from ~623 nm to ~547 nm for Cd2+ and ~540 nm for Hg2+, and the kpeak of mode2 changed from 664 nm to 514 nm and 502 nm for Cd2+ and Hg2+, respectively (Fig. 5f). The particles shrunk as pyridine groups got crosslinked

Fig. 4. (a) Hydrodynamic diameter of copolymer particles with respect to molar ratio of styrene/4-vinylpyridine (St/4VP). (b) The diffuse reflectance (Solid lines represent Gaussian fitting), (c) photographs and RGB-converted images of poly(St-co-4VP) particle dispersions with 8.0% carbon black. (d) Theoretically estimated and measured hue of Mie backscattering-based colours. Saturation and brightness of theoretical colours were set to 20%, 39%, respectively. (e) Theoretically estimated colours of mode3 and mode2 peaks of SV424 and the mixture of the two (Theoretic) compared with measured colour.

Please cite this article as: S.-J. Lee, S. Kumar, J. W. Choi et al., Coloration of colloidal polymer particles through selective extraction of Mie backscattering for cation-responsible colorimetric sensors, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.073

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Fig. 5. Schematic images showing swelling-deswelling of poly(4-vinylpyridine) (P4VP) particles in different conditions. (b) Hydrodynamic diameters, (c) wavelengths of peak reflectance, (d) diffuse reflectance, and (e) photographs and RGB-converted images of P4VP colloids in accordance with pH. (f) Hydrodynamic diameters (bars) and wavelengths of peak reflectance (symbols) of Cd2+, and Hg2+ adsorbed P4VP colloids. Error bars indicate the largest and the smallest diameter among the 70 measurements. (g) Photographs and RGB-converted images, and (h) diffuse reflectance of Cd2+, and Hg2+ coordinated P4VP colloids. Solid lines in (d) and (h) represent Gaussian fitting.

through coordination with the metal ions (Fig. 5a) [28,29]. The colours of colloids were changed from reddish-brown to green and cyan for Cd2+ and Hg2+, respectively (Fig. 5g). Although mode2 peaks of Cd2+ and Hg2+ added colloids were similar (~514 nm, ~502 nm), the revealed colours were different because particle aggregated to appear a peak at the shorter wavelength for the Hg2+ added colloid (Fig. 5h) [30].

4. Conclusions Mie scattering-based coloration of non-absorbing polymer particles was investigated. Due to the chemical functionality and swelling-deswelling ability of 4-vinylpyridine (4VP)-contained polymer particles, Mie scattering-based colours were able to be tuned according to the size changes of the polymer particles responding to pH and heavy metal ions. It is the first time to show the Mie backscattering-based coloration using size-tunable polymer particles. As demonstrated in the previous reports, decreasing the number of scattering is the main point to exhibit Mie backscattering-based colours from the colloidal solution [18]. Hitherto, studies on the extraction of Mie backscattering-based colour have focused on modifying the optical properties of the scatterers [19,20]. In this work, however, we modified the optical property of the media by dispersing carbon black (CB). Although the CB in the media did not affect the number of scattering, Mie backscattering-based colours were revealed because Mie forward scattering was selectively absorbed by CB due to the path length difference between Mie forward scattering and Mie backscattering. Thereby, it was able to observe Mie backscattering-based colour without modifying the optical properties of the scatterers. This study has expanded the compositional scope of scatterers that

could be used for the Mie backscattering-based coloration to non-absorbing, higher refractive index (1.1 < neff < 1.6) polymers. In addition, the Mie backscattering-based colour improved to be active that the colour could be changed by sensing pH changes and heavy metal ions. The coloration of polymer colloidal particles may have potential applications in sensors. Especially, designing polymer particles that have higher sensitivity on specific stimuli may open new opportunities to Mie backscattering-based colorimetric sensors to be applied to biosensors and environmental sensors. Acknowledgements This research was supported by ‘‘Nobel Research Project” grant for Grubbs Centre for Polymers and Catalysis funded by the GIST in 2019.

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Please cite this article as: S.-J. Lee, S. Kumar, J. W. Choi et al., Coloration of colloidal polymer particles through selective extraction of Mie backscattering for cation-responsible colorimetric sensors, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.073

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Please cite this article as: S.-J. Lee, S. Kumar, J. W. Choi et al., Coloration of colloidal polymer particles through selective extraction of Mie backscattering for cation-responsible colorimetric sensors, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.073