Organic Nanocolorants: Self-Fixed, Optothermal Resistive, Silica

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Organic Nano-colourants: A self-fixed, optothermal resistive silica supported dyes for sustainable dyeing of leather sathya ramalingam, Kalarical Janardhanan Sreeram, Jonnalagadda Raghava Rao, and Balachandran Unni Nair ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00218 • Publication Date (Web): 30 Mar 2016 Downloaded from http://pubs.acs.org on March 31, 2016

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ACS Sustainable Chemistry & Engineering

Organic Nano-colourants: A self-fixed, optothermal resistive silica supported dyes for sustainable dyeing of leather Sathya Ramalingam, Kalarical Janardhanan Sreeram, Jonnalagadda Raghava Rao*, and Balachandran Unni Nair Chemical Laboratory, Council of Scientific & Industrial Research- Central Leather Research Institute, Adyar, Chennai-6000 20, India *Corresponding Author: [email protected]; Tel. +91 44 2441 1630, Fax: +91 44 24911589 Abstract The emerging field of nanotechnology aims at revolutionizing the industrial world via introducing nano-based material for colouring. In particular, silica based colourant nanoparticles for colouring was developed, which shows extraordinary performance in colouring world dually with self-fixing property. In this regard, the use of nano based prefunctionalised colourant, which possess perfect framed structure is unexplored in colouring world especially in leather colouring along with stabilisation. Herein, we report the inclusion of organic cationic colourant into the silica matrices by simple emulsion technique in aqueous condition. Unique photophysical properties displayed by the colourant enable the development of newer colouring method, and excellent resistance to high optical and thermal conditions. In addition, the newer system does not require any pre-treatment like acidification for fixing the colourant with minimum amount of water usage. It penetrates and fixes in to the matrix without any need of colouring auxiliaries, thereby halved the environmental pollution load. Excellent stability from the new method of silica functionalised colouring system will not only reduce the environmental burden but also make the colouring process to be sustainable. It will also highlight some noteworthy recent avenues in using nanometre sized materials as a colouring agent for processing fibrous matrix and also in colouring cellulose in textile, keratin in hair dyeing, emulsion in paint industry. Finally, the developed nanoparticles containing silica based colourant with superior colouring properties forms an important area of research with significant prospects for colouring application. Keywords: Leather dyeing, Nano-dyes, Silica colourants, Self-fixing, Stable colourants Introduction Among the most promising systems, nanomaterials are currently attracting the worldwide interest of researchers to develop novel approaches in biotechnology,1 leather2, textile industry3, paint & pigment, and hair dyeing etc. The possibility of combining different modalities in a single nanoparticle makes them very attractive for various applications. Recently, the research interest have been directed towards incorporating nanotechnology into industrial applications, which enables the industry to meet stricter legislation regarding environmental safety.4 Attempts have been forwarded towards the uses of nanoscale material for processing fibrous skin matrix at different stages like nanocomposites in tanning5, hybrid 1 ACS Paragon Plus Environment


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nanomaterials in retanning6, nano based metal coating in finishing7 and also in effluent treatment by using nanofilters.8 But very minimal work contributes to the application in colouring of fibrous matrix like uses of nanocomposite as additive for colouring. Colouring adequate matrix has prominent application in textile, paint, and hair dyeing etc. Leather, fibrous matrix with collagen protein is coloured for value addition, according to customer’s desire with enhancement in its appearance. In general, the process of colouring is conventionally done by employing anionic, direct, sulphur, cationic or metal complex colourants, the choice of which is limited by the nature of the stabilising material used.9 Currently 90% of the skin matrix was stabilised by using Cr(III) metal salt. Cr(III) crosslinked skin matrix predominately carries positive charge and for this reason it can be coloured directly with anionic dyes for better penetration, whereas it exhibits practically no capability of combining with cationic colourant. In addition, the surface fixation is favoured under acid conditions due to protonation of amino groups in the collagen matrix. Hence, the binding forces between the dye and protein present in the skin matrix are relatively weak and little reversible.10-12Besides, adsorbed colourant molecules in the peripheral regions of the coloured fibrous matrix will readily diffuse out of the substrate during wet processing. Predominantly, anionic colourants hold higher rate with disadvantage of poor stability against wet and dry rub with less surface colouring. In many cases the uniform adsorption and penetration of free anionic dyes is not yet satisfactory, and the suede type of leather matrix is then topped with free cationic dyes.13Similarly, high surface coloured fibrous matrix is achieved by using cationic colourants with suitable platform. Unsatisfactory performance of cationic colourants in penetration of the substrate minimise this usage in leather colouring especially to chrome stabilised leather. Collectively, weakness of these electrostatic attractions and reversibility of the dyeing mechanism makes the conventional dyeing with less uptake leading to discharge of highly coloured wastewater streams. A variety of attempts like pre-impregnation14 pigment-dye system, functionalized dye, auxiliary agents containing metal salt,15and dye levelling agent are typically developed to improve the penetration property, resistivity with uniform adsorption. However, these attempts induce instability towards other auxiliaries like high polymeric syntans, self-emulsified fatliquors and acidbase fixing auxiliaries with incremental pollution load. Further inspite of these, the challenges with respect to charge of the leather matrix still remains and type of dye that can be used is questionable. In consideration of the problem described above, recent research continuously dedicate towards searching for a high-efficient stable colouring system with integrated merits. Binding of colourant towards stabilised matrix without being influenced by the respective charges or other auxiliaries employed as a part of post tanning is not currently feasible. This is time for the tanner to revolutionise the process by adopting newer technology like nanotechnology. In this sense, silica based nanoparticle colourant is regarded as one of the most promising material for colouring fibrous leather matrix due to their unique characteristics, namely size,16 chemical stability,17photo-stability18and versatility as well as low toxicity towards biomolecules.19 The controlled entrapment of dye inside the silica matrixes attracted greater interest owing to their superior property.20 The use of dye in nanoscale provides freedom to tweak the fundamental properties such as solubility, diffusivity, permeability into 2 ACS Paragon Plus Environment

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the skin matrix with superior property than bulk. A detailed survey of the literature21-29 also indicates that the use of nano silica based colourant hitherto unexplored in colouring fibrous matrix seems to be the way forward to overcome a significant number of challenges faced by the leather sector with respect to colouring. In order to simplify the complicated conventional colouring system, herein for the first time we have reported the synthesis and application of newer nano-silica based colourant for colouring Cr(III) stabilised skin matrix. Utilisation of unique photo-physical and chemical properties rendered by silica functionalised dye nanoparticles has promoted development of novel dyeing system. In general, cationic colourants are poor in dyeing with lower binding capacity, while the silica based cationic colourant displayed high stability with precisely controlled size distribution, geometry, and surface chemistry. Nano-sized silica colourants, in compiling an ability to give uniform penetration, easy adhesion and significant biocompatibility towards dyeing, have gained much interest. The entrapment of dyes with an appropriate shell of silica results in stable framed architecture against outer environment that is suitable for leather dyeing with high stability towards light and heat. This facile pristine method of dyeing using nano silica based colourant will provide important insight for the fabrication of new colouring system with potential application in leather dyeing. Strongly, the advent of nano based silica materials would revolutionised the field of leather processing in a greener way. Experimental Section General All chemicals used were as received from supplier. Tetraethylorthosilicate (TEOS), aqueous ammonia (NH4OH) solution (25 %), Organic dye Safranin O content ≥85 %, and Surfactants Sodium Dodecyl Sulfate (SDS) were purchased from Sigma Aldrich. For dyeing trials, chrome stabilised fibrous leather matrix (goat wet blue) was taken as a source material for fibrous matrix. Chemicals used for processing fibrous matrix were of commercial grade. Silica functionalised organic colourant preparation A modified standard procedure is as follows: Micellar emulsions are formed by transferring the sodium dodecyl sulfate (0.1M) to a double–neck round bottom flask in 250 ml of aqueous mixture, which have highly tailorable systems that containing nanometer-sized water droplets stabilized by a surfactant. The micellar system essentially act as ‘‘nanoreactors’’ that assist in controlling the kinetics of particle nucleation and growth. The size and number of micelles within the emulsion system can be tuned by varying the water to surfactant ratio (W). Safranin colourant (78.939 mg) dispersed in water (2.171 mL) was added to the reaction vessel with constant stirring for a period of 10 minutes to get a homogeneous coloured solution. To the vigorously stirring, TEOS (2.5 mL) was added drop wise. Generally TEOS is chosen as a silica precursor because of its simplicity characteristics such as low cost, versatility and control over nanoparticle dimensions. In addition to the reaction mixture, 1.5 mL of NH4OH was added to initiate the polymerisation process. In this strategy, TEOS hydrolysis and nanoparticle formation are promoted by ammonia and water using ethanol as 3 ACS Paragon Plus Environment


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the TEOS co-solvent, here NH4OH act as both reactor and catalyst for the formation of silica nanoparticles. The reaction was allowed to stir for overnight followed by addition of ethanol to break the microemulsion. The particles were washed couple of times with ethanol and finally washed with water. To remove the unreacted monomers both dialysis and centrifugation were carried out. After the successful preparation of silica functionalised colourant nanoparticles, the optical properties were then investigated and the corresponding graphs are shown in Fig. 1. Transmission electron microscope provided the clear spherical shape of silica nanoparticles with organic colourant. These nanoparticles are highly dispersed than constituent dye, which are monodispersed in solution and show improved photostability compared to constituent dye. In comparison with free colourant, the Plasmon absorption maximum of the organic silica colorant nanoparticles red shifted about 530 nm from 510 nm, owing to the dye size and a relative high refractive index of the silica environment. By examining the photophysical behaviour like surface charge and hydrodynamic diameter, silica colourant are superior to aggregated conventional free colourant, making silica based colourant an attractive alternatives for application in dyeing as a colourant. Finally, application of nano silica based colourant in dyeing of leather fibrous matrix has been presented. Evaluation of organic silica based colourant Initially, the surface defect free conventionally made Cr(III) stabilised skin matrix was taken as a source material for colouring trials. Newer colouring was followed by using silica based colourant instead of free colourant. In addition the broad range pH stability of new colourant makes it possible use at any stage of leather processing as a colourant. For matched pair comparison the above mentioned full scale fibrous matrix was cut into two halves along the backbone (centre portion), in which a right half was chosen to treat with free colourant and the left half was taken for treatment with organic silica based colourant. The material to solution ratio was fixed at 1:20 based on w/v ratio. The equal amount of free colourant and silica based colourant was offered after the UV- spectrophotometer measurement. The dyeing was carried out in a rotating vessel at 27˚C for 60 min at a speed of 12 RPM (rotation per minute). No acid treatment for surface fixing of dye was done. At the end of dyeing the samples were removed from the bath and dried in the open air. The cross section of the dyed crust was taken for visual assessment for penetration of the dye. According to Haroun30 a novel method to calculate the degree of layer wise dye distribution by using light microscope compact with camera kappa. The dye distribution was calculated along the cross-section of dyed samples with 2.5X magnification and 2000 mm scale as follows. The representation of the cross sectional view of cut fibrous leather sample was shown in Fig. 2.

% = ÷ × %--------- (1) % = ÷ × % --------- (2) 4 ACS Paragon Plus Environment

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Where XY is skin thickness, XZ is thickness of coloured layer from grain side and AY is the thickness of coloured layer from flesh side which is shown in Fig. 2. Stability against water, perspiration, light, heat and rubbing was analysed as per standard methods of ISO 11640, ISO 11641, ISO 20105, SLF 20 and ISO 20433 respectively. For stability against water and perspiration, coloured samples were subjected to the ISO 105:C06/A2 wash test (37˚C) using multifibre fabric as an adjacent either in water or an alkaline artificial perspiration solution. After 4 h treatment the samples are visually assessed to the ISO 105:A02 and ISO 105:A03 test protocol to determine the degree of wash down and cross staining, respectively. The grey scale ranges from 5 for no shade change (or no stain on the adjacent fibres) down to 1 for a severe shade change (or staining), with half points in between. The grey scale assessments should be made under artificial illumination. Samples were subjected to the ISO 105 B02:2014 (Source: Air Cooled Xenon Arc) (Modified) light fastness test using SDC blue wool standards 1 to 8 as reference. The blue wool scale ranges from 8 for excellent light fastness down to 1 for very poor light fastness. To further demonstrate the colour stability against hot surface the coloured matrix were subjected to short term contact with hot surfaces like 50˚C, 100˚C, 150˚C, 200˚C and 250˚C, to determine the amount of visible damage in terms of colour change and surface defects with reflectance measurement. Resistivity against the frictional force was to assess the degree of damage (marring) and transfer of colour onto the cotton during wet and dry abrasion. The test is pre-set for predetermined number of revolutions and the damage or transfer of colour is assessed subjectively using grey scale. In similar way the colour fastness to rubbing in crockmeter was followed. Dye penetration and leaching studies To know the compatibility of dyes towards various pH conditions, leaching of the unbound dyes was investigated. The coloured matrix presented here consists of the unbound dyes, without any chemical conjugation. Thus there may be a tendency of the dye to leach out from the matrix during different physiological conditions (pH like 1.8, 2.7, 3.8, 4.9 and 5.8). In order to check for any dye leaching the 6 grams of small cut piece of coloured material was suspended in above mentioned pH solutions and stirred for 48 h, at constant volume. Periodically the filtrate was collected and the concentration of the dye was determined by comparing the absorbance of filtrate with absorbance of a known concentration of dye. The absorption studies were done using Schimadzu UV-Vis spectrometer. In-vitro collagen stability studies To study the in vitro stability of pure collagen (skin protein), 90 µL of collagen solution @ pH 4.3 was incubated with 16.5µL of both free colourant and colourant entrapped silica nanoparticles at 37˚C for 48 h. At each time point, a small fraction of the mixture (15µL) was collected, and re-suspended in 100µL of PBS (Phosphate Buffered Saline). The kind of bonding and conformational change were measured by using ABB MB3000 Fourier 5 ACS Paragon Plus Environment


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transform infra-red (FTIR) spectroscopy and Circular Dichroism (CD) spectropolarimetry. All FTIR spectra were performed with a resolution of 4 cm-1 and recorded at 45˚incident angle using Calcium fluoride crystal plate in the region 4000 to 600 cm-1. Circular dichroic spectra were measured using a Jasco 715 Circular Dichroism spectropolarimeter using a quartz cell with a light path of 1mm at 0.2 nm intervals, at 25 ˚C, with 2 scans averaged for each sample. Result and discussion In order to avoid the instability caused by changes in the material properties, a well characterised dye entrapped 50 nm silica nanoparticle was used in this study as colouring agent. The particle dispersity was analysed by the Transmission Electron Microscope (TEM) to rule out the presence of any larger aggregates that could affect the penetration into porous leather matrix. At higher magnification the colouring matter visible as small dark spots embedded inside the silica spheres as shown in Fig. 1 (a), suggesting better penetration into fibrous matrix than constituent dye. Furthermore, Zeta potential measurements showed that dye entrapped nanoparticles were highly negative charged (- 48.7±1mV), indicating a stable suspension in aqueous medium. To demonstrate their potential application as a colouring agent, we have investigated their dyeing properties by using them for colouring of Cr(III) stabilised skin matrix. Here the skin protein, collagen was stabilised by Cr(III) complexes with positive charge. First the positive charged skin matrix (@ pH 4) was treated with free cationic dye (600nm size, polydispersed Fig. 1 c.) for 1hr, exhibited surface colouring with invisible dye penetration (Fig. 3 a). In conventional colouring of Cr(III) stabilised skin matrix, assumption has made that only anionic agent have a capability to bind with chrome stabilised proteinous skin matrix due to variation in surface charges available for binding.23 Interestingly, the uniform colouring was observed for non-conventional system of nano-silica based colourant. Instead of uneven colour distribution, uniform colour value in layer wise analysis was obtained for silica-functionalised dyes (Fig. 3 c and d) with uniform cross sectional colouring (Fig. 3b). For example colour value (a*) of nano-silica based coloured fibrous matrix with grain layer = 21, middle layer = 19, flesh layer = 22, respectively and the colour value (a*) of free cationic coloured fibrous matrix with grain layer = 22, middle layer = 13, flesh layer = 5 respectively (Fig. 3a). It is evident from the difference in colour value that free colourant contributes nonuniform distribution of dyes. Further, repeated sets of colouring experiments using free colourant, presented the difference in colour shade of final coloured fibrous matrix, and while the uniform coloured pattern was obtained in silica based colourant. Reason for non-uniform adsorption of free cationic dye could be due to surface charge repulsion and poor solubility in aqueous medium. Free cationic colourants generally exist as larger particles or aggregated lumps due to its intensive molecular interaction and crystalline property.11 But the inclusion of dye in a silica nanoparticle was an elegant way to make them highly soluble in aqueous medium. Dye with framework structured silica shell having multiple hydroxyl groups lead to high diffusion and uniform penetration without influenced by the surface charge (Fig. 4). This is supported by ATIR study for the interaction between the silica nanoparticles and pure collagen (skin protein) (ESI Fig. S1). To further demonstrate the role of silica while binding 6 ACS Paragon Plus Environment

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of colourant, the interaction studies were carried out after incubation with native collagen protein (type I). The perturbations in secondary structure are confirmed by difference between ATIR spectrum of pure collagen and its interactions with dyes (Free and Silica based). The characteristics peaks of pure collagen were changed from their positions, with appearance of new peaks when colourant entrapped silica nanoparticles were attached to collagen surface. The amide I band, characteristics of C=O stretching vibrations of pure collagen, appears with a position changed from 1634 cm-1 to 1640 cm-1. Similarly, amide II band, C-N stretch and N-H bending of pure collagen shifting from 1565 cm-1 to 1574 cm-1 because of their interactions with silica nanoparticles.31 Chemical shift in this region from 800 to 1200 cm-1 could indicate the rearrangement of quasi structural water of the collagen hydration shell and formation of water bridges between the collagen and negative silica surfaces.32The results mentioned above indicate that collagen interacted to the silica nanoparticles can be regarded as a proof of the H-bonding between them. The appearance of new peak at 1227 cm-1 is assigned to internal asymmetric Si-O stretching mode of silica nanoparticles.33Finally the results clearly indicate the involvement of more number of hydroxyl groups of silica nanoparticles for interaction with collagen skin protein stabilise the fibrous matrix. Further high colour values indicate that the nano-silica based colourant has superior ability to permeate through the skin matrix and multi-crosslinking with collagen, in contrast to single – dye interaction. The chrome complexes and protonated carboxyl groups containing chrome tanned leather have been found to have high affinity for silica34 based colourants that display a negative surface charge due to the presence of deprotonated multiple hydroxyl groups on the surfaces.35 Such adsorption of colourant was proven by uniform colour value obtained in layer wise (grain, middle, flesh) analysis of coloured leather. To assess whether uniform adsorption of colouring is due to smaller particle size or available functional groups, degree of dye adsorption was performed on each coloured layer (Fig. 5 c and d). Results showed that both particle size and framework structure silica containing functional groups have combined effect in adsorption properties, which is supported by less variation in dye distribution over the entire matrix thickness, where difference in fibre orientation being observed. The result obtained is superior to other reported systems.12,23The fabrication of silica based colourant involved microemulsion formation using Sodium dodecyl sulphate (SDS) as a template. The presence of SDS acts as a dispersing agent for better diffusion and adsorption of silica based colourant over the leather matrix.36 The improvement in adsorption of dyes at fibrillar collagen arises from the presence of smaller particle in silica based colourant, thus allowing silica colourant to penetrate easily and approach the collagen at fibrillar level, establishing multiple hydrogen bonding.37 This agrees well with CD measurements of collagen after interaction with free and silica nanoparticles containing colourant ESI Fig. S2. Whilst in the case of collagen interacted with colourant entrapped silica nanoparticles where the triple helix structure was completely preserved, the negative absorbance band intensity decreased 38,39compared to unpreserved secondary structure of free colourant system. This feature suggests that the introduction of negative surface silica nanoparticles with hydroxyl functional group stabilise the secondary structure of collagen through hydrogen bonding. The collagen silica binding is very complex to predict and as emphasized in this work it depends on many variables. On the basis of 7 ACS Paragon Plus Environment


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experimental results we propose a binding model, for collagen and silica based colourant involving hydrogen bonding, shown in Fig. 6. This model is strongly represented with an availability of functional sites with their relative chances of binding sites. In this hydrogen bonding between single unit of collagen and silica nanoparticles are represents in yellow circles. Both results provided evidence that the newer dyeing system of colourant entrapped silica nanoparticles binding to collagen skin protein through hydrogen bonding. Though uniform adsorption of colouring is achievable by silica based colourant, NPs should be stable over a range of pH conditions for their effective function as leather colouring agent. To validate further, charge of the fibrous matrix is modified with various pH conditions of 5, 6, and 7. Effect of pH on leather colouring doesn’t show much change in final dyeing characteristics and the dye distribution was coincident with that obtained at pH 4 conditions. These results support that dyes in silica frame network are less sensitivity to physiological conditions due to their decreased interactions with the outer environments.40 It is worth here to compare the ability to surface fix free cationic dye and silica based cationic dye without any auxiliaries. It is still a large challenge to fix the dyes on leather surface without any pre-treatment. To fix the dye, surface acidifications are needed to protonate the surface charge and that have electrostatic interaction with free colourant. In contrast the stable physiological property and multiple functional site of silica based colourant, fix on to the surfaces indeed of pre-treatment. Avoidance of hard to destroy grade chemicals direct the silica based colourant into the greener path in a sustainable way. The preliminary requirement for coloured matrix is the need to strike a balance between uniform adsorption and surface fixation with excellent stability against physiological conditions like heat, light, water and artificial perspiration solution. To understand the resistivity like colour transfer against water and perspiration solution, the coloured matrix are exposed to water and artificially prepared perspiration solution. Grey scale rating showed that all adjacent fabrics encountered less colour change, and it is due to the framed silica structure readily absorb the unfixed dye generated (Table 1). In the presence of high functional silica network over the nanoparticle surface reducing the silica- protein bond cleavage with superior stability against water and perspiration solution. Stability towards light and heat were also tested by using the artificial light and heat. Photostability is also important when the matrix is as high end application in complex physiological environments. Fig. 5b shows the photobleaching behaviour of the nanoparticles and coloured matrix. As expected, after exposure to artificial light irradiation (UV lamp (365 nm) for solution and 270 watt Xenon arc lamp light for coloured matrix) for 60 min, the silica based colourant displayed reduced photobleaching41 compared to free colourant. Similarly free colourant coloured matrix is highly susceptible towards photo-oxidation compared to silica based colourant Less bimolecular interaction between the colourant and reactive species (such as oxygen) in silica based colourant shows superior photobleaching property than free colourant 42,43 (Scheme 1). Mechanism for higher stability towards light was attributed to preventing oxygen molecule penetration into the nanoparticles to react with the dye molecules.44 In addition the colourant surface is never exposed to reaction medium and thus less prone to oxidation and chemical treatment.45 8 ACS Paragon Plus Environment

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In order to study the stability against the heat, selected coloured matrix was exposed to metallic hot surfaces for 5 seconds. The experiment was done at different temperatures like 50, 100, 150, 200, and 250˚ C (Table 2), to differentiate the rate of colour change. Increase in L (lightness) value of free colourant adsorbed coloured leathers indicate the breaking of ionic interaction between the dye and matrix due to supplied heat. Considering that the silica framed colourant could not be affected by heat, because energy gets dissipated due to high energy required to fragment the high network structure of silica nanoparticles.46-48 As shown in Fig. 8, silica nanoparticles led to a significant increase in heat and light resistance, indicating that the pristine silica based colourant system will act as novel for dyeing fibrous matrix especially leather. These results were further supported by stability against frictional force with less transfer of colour in silica nanoparticles. Our works include the confirmation on bleeding of residual unfixed dyes during subsequent washing with different pH solutions. The percentage of dye leached at different pH conditions was plotted and shown in the Fig. 7a. The above experimental results clearly reveal the affinity of unfixed colourant depends on individuals. Leaching of minimal amount of unfixed colourant stipulate the irreversible interaction of nano based silica colourant with leather matrix compared to conventional reversible free colourant system. Two mechanisms are proposed to describe the bleeding of colourant from the fibrous matrix. First is desorption of physically adsorbed dye out of the dyed crust. Second is the delamination of the entire dye containing layer due to its poor adhesion to the fibrous matrix. Herein, the amount of leached colourant is very minimal indicating the good interaction and penetration of dyes in the silica based colouring system. In addition the dyes entrapped inside the silica shell doesn’t have direct contact with protein that leads to less bleeding and transfer of colour. It is worth mentioning that the silica based colourant nanoparticles used in this study labelled as attractive dyeing system in a greener way. We have also investigated the property supplement induced by silica based material during colouring. The air permeability measurement clearly demonstrates the reduction in the flow rate49 mainly due to the more adsorption of colourant containing silica nanoparticles at fibrillar level collagen matrix (Fig. 7 c). The high softness value obtained in silica based colourant, mainly due to porous structure of silica responsible for increases in softness and smoothness of the grain with less uneven coating over the surface (Fig. 7d). Herein, the utilisation of silica based colourant not only provides uniform colouring but also the enhanced organoleptic properties (Fig. 7 a). Surprisingly, silica based colourant treated leathers showed a highly compact fibre orientation and smooth surface (ESI Fig. S3) shown in the SEM (Scanning Electron Microscope) image of coloured matrix. The obtained filled fibres may be attributed to the attachment of silica nanoparticles over the fibres introduce inter-fibre interactions. The enhancement in property supplement was registered for silica based colourant with the uniform dye distribution and colour shade (Fig. 7a). The results were comparable to that of reported value for novel hybrid silica nanoparticles based retanning agent4 and amino functionalised dyes,50 while providing more than high colour values with surface smoothness of the later, further demonstrating the excellent performance of the silica based colourant. The resulting coloured leathers exhibit balance of all the required properties and an enhanced resistivity compared to free colourant coloured leather. 9 ACS Paragon Plus Environment


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Conclusion and perspective In this work a novel newer system of dyeing was attained by using nano silica based colourant, which provides excellent stability, uniform penetration and surface fixation as compared to the conventional free colourant system. To our knowledge this is the first attempt has been made in leather science to develop the nano sized silica based organic colourant for colouring along with stabilisation. Self- fixed dyeing system eliminates the process of acidification thereby minimising the pollution load. Poor penetration and bronziness are the major drawbacks of free cationic colourant in dyeing of chrome tanned leather, but narrow size distribution and high surface charge of the newer system enable deeper penetration with less surface deposition. In this new method of dyeing, silicon like unit over the dye containing large number of Si-OH group was chosen to favour multi hydrogen bonding, while the presence of surface hydroxyl was considered to be suitable for dyeing of any charged matrix without any specification. The resulting coloured leathers exhibit balance of all the required properties and an improved resistivity compared to conventionally coloured fibrous matrix. Uniform colour value in the cross section of the coloured matrix is due to their small size and predicted multiple bonding of the silica based colourant. The colourant inside the silica nanoparticles are stable and there is no direct contact between the colourant and protein present in the skin matrix, which leads to less bleeding and transfer of colour with high stability towards heat and light. The silica based colourant was introduced as pristine colouring system to produce uniformly coloured matrix with excellent stability against high optical and thermal conditions. In this work it has been successfully validated the approach of dye entrapment into silica matrix to maintain better dyeing conditions for novel colouring of leather fibrous matrix. This silica colourant will act as a better replacement for dyeing agent in textile, pigment and hair dyeing industry. This approach can be generalised to other kind of dyes by selecting appropriate linkage between outer shell platform and core dye. In the follow up studies, we have planned to functionalise the silica for better dyeing systems. The anticipated developments in the synthesis and surface modification in the future provide possibilities for newer dyeing system through covalent bonding with leather matrix and broadening of application scopes. Acknowledgment The authors thank the financial support of Supra Institutional Project STRAIT CSIR. One of the author R. Sathya wishes to thank the DST-INSPIRE, New Delhi, for providing a Junior Research Fellowship.

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16. Kumar, R.; Roy, I.; Ohulchanskyy, T. Y.; Goswami, L. N.; Bonoiu, A. C.; Bergey, E. J.; Tramposch, K. M.; Maitra, A.; Prasad, P. N., Covalently Dye-Linked, SurfaceControlled, and Bioconjugated Organically Modified Silica Nanoparticles as Targeted Probes for Optical Imaging. ACS Nano 2008, 2, (3), 449-456. 17. Lee, J. E.; Lee, N.; Kim, T.; Kim, J.; Hyeon, T., Multifunctional mesoporous silica nanocomposite nanoparticles for theranostic applications. Acc Chem Res 2011, 44, (10), 893-902. 18. Cohen, B.; Martin, C.; Iyer, S. K.; Wiesner, U.; Douhal, A., Single Dye Molecule Behavior in Fluorescent Core–Shell Silica Nanoparticles. Chem. Mater. 2012, 24, (2), 361-372. 19. Hurley, M. T.; Wang, Z.; Mahle, A.; Rabin, D.; Liu, Q.; English, D. S.; Zachariah, M. R.; Stein, D.; DeShong, P., Synthesis, Characterization, and Application of Antibody Functionalized Fluorescent Silica Nanoparticles. Adv. Funct. Mater. 2013, 23, (26), 3335-3343. 20. Prichystal, P.; Burgert, L.; Hrdina, R.; Purev, N.; Cerny, M., Encapsulation of textile dyes and textile auxiliary agents into liposome systems and their use for polyamide dyeing. Color. Technol. 2013, 129, (1), 64-68. 21. Motamedian, F.; Broadbent, A. D., Effects of Dye Distribution in Nylon Filament Yarns on the Dyeing Color Yield and Fastness Properties. Ind. Eng. Chem. Res. 1999, 38, (12), 4656-4662. 22. Sun, S.-S.; Xing, T.; Tang, R.-C., Simultaneous Coloration and Functionalization of Wool, Silk, and Nylon with the Tyrosinase-Catalyzed Oxidation Products of Caffeic Acid, Ind. Eng. Chem. Res. 2013, 52, (26), 8953-8961. 23. Sivakumar, V.; Rao, P. G., Power ultrasound-assisted cleaner leather dyeing technique: influence of process parameters. Environ. Sci. Technol. 2004, 38, (5), 1616-21. 24. Blackburn, R. S.; Harvey, A., Green chemistry methods in sulfur dyeing: application of various reducing D-sugars and analysis of the importance of optimum redox potential. Environ. Sci. Technol. 2004, 38, (14), 4034-9. 25. Haroun, A. A., Mansour, H. F., New approaches for the reactive dyeing of the retanned carbohydrate crust leather. Dyes Pigments. 2008, 76, 213-219. 26. Haroun, A. A.; Mansour, H. F., Effect of cationisation on reactive printing of leather and wool. Dyes Pigments. 2007, 72, (1), 80-87. 27. Sivakumar, V.; Swaminathan, G.; Rao, P. G.; Muralidharan, C.; Mandal, A. B.; Ramasami, T., Use of ultrasound in leather processing industry: effect of sonication on substrate and substances--new insights. Ultrason. Sonochem. 2010, 17, (6), 10549. 28. Sivakumar, V.; Rao, P. G., Studies on the use of power ultrasound in leather dyeing. Ultrason. Sonochem. 2003, 10, (2), 85-94. 29. Dyeing method of acid dyes assisted by functionalized carbon nanotubes. In Google Patents: 2014.


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30. Haroun, A. A., Evaluation of modified leather dyeing technique using black dyestuffs from the economical view. Dyes Pigments. 2005, 67, (3), 215-221. 31. Eklouh-Molinier, C.; Happillon, T.; Bouland, N.; Fichel, C.; Diebold, M. D.; Angiboust, J. F.; Manfait, M.; Brassart-Pasco, S.; Piot, O., Investigating the relationship between changes in collagen fiber orientation during skin aging and collagen/water interactions by polarized-FTIR microimaging. Analyst 2015. 32. Huggins, M. L., Hydrogen bonding in high polymers and inclusion compounds. J. Chem. Educ. 1957, 34, (10), 480. 33. Brassard, J.-D.; Sarkar, D. K.; Perron, J., Synthesis of Monodisperse Fluorinated Silica Nanoparticles and Their Superhydrophobic Thin Films. ACS Appl. Mater. Interfaces. 2011, 3, (9), 3583-3588. 34. Horiuchi, Y.; Fujiwara, K.; Kamegawa, T.; Mori, K.; Yamashita, H., An efficient method for the creation of a superhydrophobic surface: ethylene polymerization over self-assembled colloidal silica nanoparticles incorporating single-site Cr-oxide catalysts. J. Mater. Chem. 2011, 21, (24), 8543-8546. 35. Puddu, V.; Perry, C. C., Peptide Adsorption on Silica Nanoparticles: Evidence of Hydrophobic Interactions. ACS Nano 2012, 6, (7), 6356-6363. 36. Tong, W.; Zhu, Y.; Wang, Z.; Gao, C.; Mohwald, H., Micelles-encapsulated microcapsules for sequential loading of hydrophobic and water-soluble drugs. Macromol. Rapid. Commun. 2010, 31, (11), 1015-9. 37. Tang, L.; Cheng, J., Nonporous silica nanoparticles for nanomedicine application. Nano Today 2013, 8, (3), 290-312. 38. Mandal, A.; Sekar, S.; Chandrasekaran, N.; Mukherjee, A.; Sastry, T. P., Synthesis, characterization and evaluation of collagen scaffolds crosslinked with aminosilane functionalized silver nanoparticles: in vitro and in vivo studies. J. Mater. Chem. B 2015, 3, (15), 3032-3043. 39. Sapsford, K. E.; Algar, W. R.; Berti, L.; Gemmill, K. B.; Casey, B. J.; Oh, E.; Stewart, M. H.; Medintz, I. L., Functionalizing Nanoparticles with Biological Molecules: Developing Chemistries that Facilitate Nanotechnology. Chem. Rev. 2013, 113, (3), 1904-2074. 40. Ow, H.; Larson, D. R.; Srivastava, M.; Baird, B. A.; Webb, W. W.; Wiesner, U., Bright and Stable Core−Shell Fluorescent Silica Nanoparticles. Nano Lett. 2005, 5, (1), 113-117. 41. Aznar, E.; Marcos, M. D.; Martínez-Máñez, R.; Sancenón, F.; Soto, J.; Amorós, P.; Guillem, C., pH- and Photo-Switched Release of Guest Molecules from Mesoporous Silica Supports. J. Am. Chem. Soc. 2009, 131, (19), 6833-6843. 42. Hau, F. K.-W.; Lee, T. K.-M.; Cheng, E. C.-C.; Au, V. K.-M.; Yam, V. W.-W., Luminescence color switching of supramolecular assemblies of discrete molecular decanuclear gold(I) sulfido complexes. Proceedings of the National Academy of Sciences of the United States of America 2014, 111, (45), 15900-15905. 43. Sun, B.; Pokhrel, S.; Dunphy, D. R.; Zhang, H.; Ji, Z.; Wang, X.; Wang, M.; Liao, Y. P.; Chang, C. H.; Dong, J.; Li, R.; Madler, L.; Brinker, C. J.; Nel, A. E.; Xia, T., Reduction of Acute Inflammatory Effects of Fumed Silica Nanoparticles in the Lung by Adjusting Silanol Display through Calcination and Metal Doping. ACS Nano 2015. 13 ACS Paragon Plus Environment


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44. El-sayed N. H.; Haroun, A. A.; Stoll, M., Investigation of tartaric acid\copper sulfate as light stabilizer in leather dyeing. JSLTC 2003, 87, (4), 138-143. 45. Wang, L.; Wang, K.; Santra, S.; Zhao, X.; Hilliard, L. R.; Smith, J. E.; Wu, Y.; Tan, W., Watching Silica Nanoparticles Glow in the Biological World. Anal. Chem. 2006, 78, (3), 646-654. 46. Joo, S. H.; Park, J. Y.; Tsung, C.-K.; Yamada, Y.; Yang, P.; Somorjai, G. A., Thermally stable Pt/mesoporous silica core-shell nanocatalysts for high-temperature reactions. Nat. Mater. 2009, 8, (2), 126-131. 47. Arabli, V.; Aghili, A., The effect of silica nanoparticles, thermal stability, and modeling of the curing kinetics of epoxy/silica nanocomposite. Adv.Compos.Mater. 2014, 1-17. 48. Veith, G. M.; Lupini, A. R.; Rashkeev, S.; Pennycook, S. J.; Mullins, D. R.; Schwartz, V.; Bridges, C. A.; Dudney, N. J., Thermal stability and catalytic activity of gold nanoparticles supported on silica. J. Catal. 2009, 262, (1), 92-101. 49. Ramalingam, S.; Janardhanan Sreeram, K.; Raghava Rao, J.; Unni Nair, B., Hybrid composites: amalgamation of proteins with polymeric phenols as a multifunctional material for leather processing. RSC Adv. 2015, 5, (42), 33221-33232. 50. Fennen, J.; Page, C. T.; McHugh, J. L., Dyed leather and method for dyeing tanned leather. In Google Patents: 2005.


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Figure Captions Fig. 1 HR-TEM images (a, b) of silica based colourants, and hydrodynamic diameter (c) of the free colourant and silica based colourant, and their corresponding Plasmon absorption maximum (d). Fig. 2 The cross- sectional view of coloured fibrous leather matrix Fig. 3 The photomicrograph contains cross sectional view of (a) free colourant and (b) silica based colourant coloured fibrous leather matrix, (c and d) are the photomicrograph of corresponding sectional layers. Fig. 4 The schematic representation of binding mechanism of free and silica framed colourant. Fig. 5 a) Colour measurement – a* value for free colourant and silica based colourant coloured leather, b) Photostability of free and silica based colourant, (c and d) degree of penetration of dyes from grain (c) and flesh (d) side in obtained from different physiological conditions respectively. Fig. 6 Predicted mechanism of interaction of skin protein collagen (Triple helical unit) with silica functionalised colourant through hydrogen bonding. Fig.7 a) Percentage dye leakage at various pH ranges, b) The comparison of supplement properties obtained for free colourant and silica based colourant treated matrix, c) permeability measurement and d) Softness imparted by free and silica based colourant. Fig. 8 a) colour swatches of coloured matrix at different temperature, b) Rub fastness and light fastness, grey scale rating of conventional free colour system and novel silica based colourant system. Scheme 1: Schematic representation of stability towards light of free and silica based colourant. Table 1: Stability towards water and perspiration solution, grey scale rating of conventional free colour system and novel silica based colourant system. Table 2: Stability against heat and their corresponding reflectance measurements are displayed.



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Fig. 1 HR-TEM images (a, b) of silica based colourants, and hydrodynamic diameter (c) of the free colourant and silica based colourant, and their corresponding Plasmon absorption maximum (d).


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Fig. 2 The cross- sectional view of coloured fibrous leather matrix



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Fig. 3 The photomicrograph contains cross sectional view of (a) free colourant and (b) silica based colourant coloured fibrous leather matrix, (c and d) are the photomicrograph of corresponding sectional layers.


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Fig. 3 The schematic representation of binding mechanism of free and silica framed colourant.



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Fig. 5 a) Colour measurement – a* value for free colourant and silica based colourant coloured leather, b) Photostability of free and silica based colourant, (c and d) degree of penetration of dyes from grain (c) and flesh (d) side in obtained from different physiological conditions respectively.


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Fig. 6 Predicted mechanism of interaction of skin protein collagen (Triple helical unit) with silica functionalised colourant through hydrogen bonding.



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Fig.7 a) Percentage dye leakage at various pH ranges, b) The comparison of supplement properties obtained for free colourant and silica based colourant treated matrix, c) permeability measurement and d) Softness imparted by free and silica based colourant.


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Fig. 8 a) colour swatches of coloured matrix at different temperature, b) Rub fastness and light fastness, grey scale rating of conventional free colour system and novel silica based colourant system.



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Scheme 1: Schematic representation of stability towards light of free and silica based colourant.


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Table 1: Stability towards water and perspiration solution, grey scale rating of conventional free colour system and novel silica based colourant system. Type of fabrics

Cellulose acetate Bleached cotton Spun nylon Spun polyester Spun acrylic Worsted spun wool

Resistance to water

Resistance to perspiration

Free colourant 4

Silica-colourant 4/5

Free colourant 2

Silica- colourant 4/5

3/4

4/5

2

4

3/4 3/4 4 3/4

4 4/5 4/5 4/5

2 3 2/3 1/2

4 4 4 4



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Table 2: Stability against heat and their corresponding reflectance measurements are displayed. Samples

Temperature (˚C)

50 100 150 200 250 Silica- colourant 50 100 150 200 250 Free colourant

a* 67.092 52.668 36.570 25.090 12.075 54.710 52.722 42.699 44.786 43.073

Reflectance b* -2.306 -6.122 -7.544 -3.201 -2.263 -8.027 -5.393 -6.122 -8.274 -8.925


L 60.013 73.196 76.028 79.769 88.325 67.148 68.193 72.175 74.324 73.499

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TOC Organic Nano-colourants: A self-fixed, optothermal resistive silica supported dyes for sustainable dyeing of leather Sathya Ramalingam, Kalarical Janardhanan Sreeram, Jonnalagadda Raghava Rao*, and Balachandran Unni Nair

Avoidance of hard to destroy grade chemicals direct the silica based nano-colourant into the greener path in a sustainable way.


Organic Nanocolorants: Self-Fixed, Optothermal Resistive, Silica

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