Soft Lithographic Patterning of Luminescent Carbon Nanodots

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Functional Nanostructured Materials (including low-D carbon)

Soft Lithographic Patterning of Luminescent Carbon Nanodots Derived from Collagen Waste Ashokkumar Meiyazhagan, Amir Aliyan, Anumary Ayyappan, Ines MorenoGonzalez, Sandhya Susarla, Sadegh Yazdi, Karina Cuanalo-Contreras, Valery N Khabashesku, Robert Vajtai, Angel A. Marti, and Pulickel M. Ajayan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13114 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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ACS Applied Materials & Interfaces

Soft Lithographic Patterning of Luminescent Carbon Nanodots Derived from Collagen Waste

AshokKumar Meiyazhagan,†* Amir Aliyan,‡ Anumary Ayyappan,† Ines MorenoGonzalez,¥ Sandhya Susarla,† Sadegh Yazdi,†#

Karina Cuanalo-Contreras,¥ Valery

N.Khabashesku,¶† Robert Vajtai,† Angel A. Martí‡ and Pulickel M. Ajayan†* †

Department of Materials Science & Nano Engineering, Rice University, Houston,

Texas, 77005, USA *Email: [email protected]; [email protected]

‡

Department of Chemistry, Rice University, Houston, Texas, 77005, USA

¥

Mitchell Center for Alzheimer’s Disease and Related Brain Disorders, Department of

Neurology, McGovern Medical School, University of Texas Health Science Center at Houston, Houston, Texas, USA.

¶

Center for Technology Innovation, Baker Hughes Inc. Houston, Texas, USA

#

Present address: Renewable and Sustainable Energy Institute (RASEI), 4001

Discovery Dr., Suite N321, UCB 027, Boulder Colorado 80309-0027

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 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

ABSTRACT

Luminescent carbon dots (Cdots) synthesized using inexpensive precursors have inspired tremendous research interest due to their superior properties and applicability in various fields. In this work, we report a simple, economical, green route for the synthesis of multifunctional fluorescent Cdots prepared from a natural, low-cost source: collagen extracted from animal skin wastes. The as-synthesized metal-free Cdots were found to be in the size range of ~1.2–9 nm, emitting bright blue photoluminescence with a calculated Cdot yield of ~63%. Importantly, the soft-lithographic method used was inexpensive and yielded a variety of Cdot patterns with different geometrical structures and significant cellular biocompatibility. This novel approach to Cdot production highlights innovative ways of transforming industrial biowastes into advanced multifunctional materials which offers exciting potential for applications in nanophotonics and nanobiotechnology using a simple and scalable technique.

KEYWORDS: Biowastes; MIMIC; Luminescence; Biocompatible; Patterning; Quantum dots


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INTRODUCTION Carbon nanomaterials have received increased attention in recent years due to their unique properties that present exciting potential for applications in vast areas of science and technology. 1-4 Important developments in the field, such as the discovery of carbon nanotubes,5 buckminsterfullerene,6 graphene,7 etc., have inspired researchers to search for the next ultimate material with unique and outstanding properties. With this in mind, carbon allotropes have been studied in detail, specifically 1D carbon nanotubes, 2D graphene, and 0D carbon dots, which have exhibited superior properties and capabilities.8-10 As a result, a new class of carbon nanomaterial, carbon nanodots or carbogenic quantum dots (Cdots), is being thoroughly studied because of its sizedependent fluorescence properties, which have important implications for materials science and nanotechnology.11-13 These emerging sp2-hybridized fluorescence Cdots usually range between 1-10 nm in size and are being considered as a potential candidate for the replacement of metal-based semiconductors14-15 and for use in a wide range of applications.16-19 One of the first reports of Cdots was documented during electrophoresis purification of single-walled carbon nanotubes.20 In general, these Cdots are synthesized using two conventional approaches, top-down (complex molecules are broken down into smaller fragments) or bottom-up (self-assembly of colloidal Cdots).21 To date, several techniques have been explored for the synthesis of an extensive range of Cdots, but

scaling-up, lack of reproducibility, toxicity of current processes and

environmental concerns are considered major roadblocks for commercialization of this material.20 Specifically, the traditional cadmium-based semiconductor quantum dots are


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toxic and expensive.22 To overcome these obstacles, researchers have developed a green chemistry strategy that utilizes bio-derived precursors as a starting material for the synthesis of Cdots.23-25 In the case of bright luminescent Cdots, raw materials used as precursor for materials synthesis included human waste (hair,26 urine27), agriculture wastes (rice husk,28 grass29), food wastes (whey,30 pomelo peels,31 orange juice,32 etc.). After careful consideration of the existing challenges presented by traditional Cdot production and inspired by previous scientific reports on green methods,

[12-18]

we

designed a simple one-step green chemistry protocol for synthesizing environmentfriendly, biocompatible, high-quality Cdots. We used protein rich collagen derived from animal skin wastes disposed from slaughterhouse and tanneries as a bio-derived precursor. Collagen is composed of a triple-helical structure, which is created by fusion of 23 different amino acids.33 Collagen was chosen for its high carbon content, biocompatibility, easy availability, and low-cost. The skins, hides and other organic matter containing collagen and a high level of proteins disposed of as wastes after slaughter for meat production are typically from livestock animals such as cattle, goats, sheep and pigs. In addition to the byproducts of meat production, the leather industries dispose of a huge quantity of proteinaceous wastes during chemical and mechanical operations. It is estimated that more than 50% of the weight of raw hides and skins are discarded.34 Hence, finding a productive use for these biomass wastes in developing advanced functional materials could bridge the gap between materials and environmental science. Most importantly, we demonstrated the possibility of fabricating micropatterns of collagen-derived Cdots using a soft-lithography technique. This technique has been


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widely studied to pattern different nanomaterials, polymers, alloys, etc. due to the simplicity of the process and flexibility of substrates.35 Most of these studies involved the patterning of Qdot/Cdots using techniques such as microcontact printing,36 inkjet printing,37 electrohydrodynamic jet printing,38 AFM lithography,39-40 etc. However, these methods have drawbacks such as (i) unpredictable shape and spacing, (ii) large areas required for patterning, (iii) complicated geometric arrays, and (iv) the high cost of instruments.41 Of note, Junkin, et al. investigated the possibility of templating colloidal Cdots using plasma lithography, but were not able to achieve complex geometrical structures over long ranges.42 After careful consideration of this study’s shortcomings, we conceived a simple technique for uniformly arranging Cdots over long range with high

precision

and

complex

dimensions

using

PDMS-based

soft-lithography.

Additionally, we were able to demonstrate the biocompatibility of the formed Cdots. The ease of synthesis, the biocompatibility of the resulting Cdots and the scalability of this technique could contribute to the fabrication of a wide range of functional devices for extensive applications in biomedicine, material science, and nanophononics. RESULTS & DISCUSSION A typical method of Cdot formation from animal skin wastes is shown in the Supporting Information (Figure S1). In general, collagen contains sequences of amino acids, the majority of which are carbon, hydrogen, oxygen, and nitrogen. In this case, elemental analysis of the hide powder used as a collagen source shows the presence of carbon (~42.3%), nitrogen (~14.9%) and hydrogen (~7.5%) (Supporting Information Table S1). We noted the shrinkage temperature of native collagen as analyzed through differential scanning calorimetry (DSC) was found to be around 63-69 °C where the triple helical


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structure starts breaking (Supporting Information S2). During the hydrothermal process, under high temperature and pressure, the macromolecular proteins were easily broken down into defect containing Cdot fragments with rich nitrogen and oxygen contents (Figure S1). The formed Cdots were then freeze-dried overnight, and the yields were calculated to be around 63%. Also, we noticed that the presence of rich functional groups increases solubility of Cdots in water and other commonly used solvents (Supporting Information S3). A detailed description of the method used for Cdot formation is discussed in the experimental section. Results from Fourier Transform Infrared (FTIR) spectrum corresponding to both collagen and Cdots is shown in Figure 1a. Both the Cdots and the collagen exhibit similar characteristic peaks around 3400, 2950, 1640, 1450, 1375, 1200 and 1070 cm-1. The broadband around 3200-3400 cm-1 corresponds to the stretching of N-H bonds in amine and the presence of O-H functionalities in hydroxyl groups.43 The significant sharp peak at around 2950 cm-1 signifies asymmetric C-H stretching of -CH3 groups and symmetric vibrations of -CH2 functional groups. Interestingly, the peak at 2950 cm-1 is observed to be more predominant compared to the precursor. This might be due to the breakdown of different amino acids present in collagen leading to the formation of more carbon rich materials. The sharp peak around 1640 cm-1 is attributed to C=O stretch of carboxyl groups and the peak at 1450 cm-1 is due to the presence of C–OH groups. The medium weak C–N peak at around 1375 cm1

is a response to the presence of primary amines, and other weak peaks at 1200 and

1070cm-1 are due to the presence of C–O, and C–O–C stretching.44 The X-Ray


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diffraction (XRD) spectrum of Cdots (Figure 1b) displays a broad peak around 23°, which could be attributed to the interlayer spacing of (002) diffraction peaks. To further confirm the presence of functional groups, the Cdots were characterized using X-ray Photoelectron Spectroscopy (XPS). The atomic composition of Cdots as revealed through the XPS survey scan (Figure 2a) was found to consist of carbon (75.2%), nitrogen (8.5%) and oxygen (16.3%). The C1s spectra (Figure 2b) exhibits three peaks around ~284, 285 and 287 eV. The peak at 284.5 eV corresponds to the presence of C-C arising due to the presence of sp2 hybridized graphitic carbon in aromatic rings.45 The peak at 285.85 eV is due to the attachment of C with O bonds and in some cases tentatively assigned to sp2 C=N and sp3 C-N bonds.46 The other peak at 287.76 eV corresponds to the presence of C=O (carbonyl) or is due to carbon bonding with nitrogen functionalities.47 The N1s spectrum (Figure 2c) displays three major peaks at 397, 399 and 401 eV. The possible bonding environments of nitrogen atoms are shown in Supporting Information S4. The peak at 397.77 and 399.58 eV corresponds to the presence of pyridine-like nitrogen atoms bonded to two carbon atoms,48 and another peak at 401.18 eV is due to the presence of quaternary or graphitic nitrogen bonded with three carbon atoms.48 These results are consistent with FTIR and CHNS elemental analysis (see Supporting Information Table S1). High-resolution transmission electron microscope (HRTEM) images (Figure 2d&e) show the crystallinity of the synthesized Cdots with a particle size of ~2 nm. Well resolved lattice fringes with an interplanar spacing of 0.32 nm closely match with d002 diffraction planes of graphite, and correspondingly the nature of crystallinity matches well with the XRD results. Atomic Force Microscope (AFM) images (Figure 2e) of the as-synthesized Cdots show particle


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size distribution between 1-10 nm (based on heights) which agrees with the TEM observations. The major percent of Cdot population (Figure 2f&g) was found to be centered around ~2 and ~6.5 nm. The primary advantage of Cdots is their simple processing, i.e., these materials can be deposited onto any substrate such as silicon, glass, plastic, etc. for fabrication of flexible devices and materials for a wide range of applications. Furthermore, Cdots are reported to exhibit excellent photoluminescence (PL), photostability, and compatibility with solution processing.49 These properties inspired us to demonstrate a simple, costeffective method to fabricate large array Cdot patterns by employing the micro-molding in capillaries (MIMIC) technique. This simple procedure is widely used to pattern micro or nano-sized arrays using polydimethylsiloxane (PDMS) stamps as molds. Moreover, this method is considered advantageous when compared to photolithography or electron beam lithography due to its low cost, easy processability, and large area fabrication.50-52 A plethora of literature reports have shown the possibility of soft lithography patterning of different materials such as fullerene,50 2D materials,51 organic perovskites,52 etc. Yet only very few studies have reported the possibility of patterning carbon dots and the reported methods were considered complex, expensive, and the formed patterns were irregular or discrete with rough surface morphologies.42,

53-54

In

this work, we demonstrate a simple technique to prepare a bulk quantity of Cdots and to organize Cdots with precise control over the different geometrical structures by adopting a MIMIC technique. The sequential organization of quantum dots, polymers or biomaterials into geometrically controlled assemblies through micro-contact patterning


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techniques is promising for an extensive range of applications ranging from nanophotonics55,56 to nanobiotechnology.57-59 Figure 3a displays a scheme for the formation of Cdot patterns using the MIMIC technique. Patterned Cdots were fabricated using PDMS stamps with channel dimensions between ~3-5 µm and height ~6 µm to form straight and complex geometrical Cdot patterns. The detailed method of fabrication of Cdot patterns was described in the experimental section. In brief, ~0.2 µL of the Cdot solution was placed at one end of a plasma treated substrate (Figure 3b), allowing the Cdot solution to flow gradually due to capillary force, thus filling the channels evenly (Figure 3c). The patterned substrates were dried for 12-14 h and the stamps were peeled off gently from the substrate. It was observed that the plasma treatment generates more hydroxyl functionalities on the surface of the substrates thus making them hydrophilic, which in turn significantly helps in strong adhesion of Cdots with the substrates. The PDMS stamps assisted as a template to self-assemble Cdots into specific geometries. Interestingly, the formed Cdot arrays were found to have the perfect geometry and dimensions similar to that of the master PDMS stamps (Figure 3d-j). The structural morphology, resolution and surface smoothness of the formed arrays were studied using optical microscopy, SEM, and AFM. Figure 3d&e show optical microscopy images of a large area of patterned Cdot arrays. The uniform color contrast observed on both optical images indicates the formation of continuous regular and smooth Cdot array/wires. Figure 3f shows low magnification (x100) and Figure 3g higher magnification (x50) SEM image of the patterned Cdots. Even under higher magnification, no defects or irregularities were observed on the surface of the patterns.


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This array formed using simple soft-lithography demonstrates the extremely precise Cdot patterns that are possible to achieve using this method. The SEM image in Figure 3g reveals the clear formation of patterns with a complete absence of residual layers. Each ring measures roughly 30 µm in size with the sidewalls around ~ 4 µm. To check the versatility of this methodology, we varied the complex geometries of the PDMS patterns and obtained similar results (Figure 3e-j & Supporting Information S5). Additionally, we observed that the patterned Cdots were stable and remained intact at room atmosphere for more than five months (Supporting Information S5), indicating significantly improved stability of the formed Cdot arrays when compared to methods previously studied. In general, the formed Cdot arrays produced using this method can be considered more advantageous when compared to the commonly reported pattern arrays due to the simplicity, easy scalability, cost-effectiveness, and high quality of the formed patterns. AFM images of the patterned Cdot array (Supporting Information S6) shows that the Cdot patterns self-aggregate to form organized, continuous nonclustered and crack-free lines. This observation supports the high binding efficiency of the Cdots, which is in strong agreement with our SEM observations. Our work demonstrates the capability for directing Cdots into designed architectures with precisely controlled geometries using a straightforward, efficient and scalable technique (See Supporting Information S5). This method introduces opportunities for the organization of complex nanostructures of multicomponent systems for a wide range of applications. A detailed PL study was carried out using different excitation wavelengths to explore the optical properties of the as-synthesized Cdots. PL of the Cdots in aqueous


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media shows an emission around 443 nm (Figure 4a). The emission slightly shifts and is broadened by decreasing the excitation wavelength. Previous studies on gelatin based particles suggested that this was due to the presence of particles with different sizes.60 Changes in particle size can lead to various extinction coefficients at varying wavelengths as well as different emission wavelengths. Thus, changing the excitation wavelength may result in exciting, diverse Cdots populations in the sample, subsequently shifting the emission maxima. This explanation is consistent with the AFM studies, where it is evident that the sample has a distribution of particle sizes, rather than a Gaussian distribution around a given size (Figure 2f&g). To further study this hypothesis, the excited state lifetime of these particles in water was recorded at 440 and 550 nm by exciting at 375 nm (Figure 4b). The decays were deconvoluted into three components with values around

Soft Lithographic Patterning of Luminescent Carbon Nanodots

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