A Comprehensive Study of 4-Sulfocalix[4]arene Thin Films with Atomic Force Microscopy: Thickness and Topographical Analysis
DOI:
https://doi.org/10.37134/ejsmt.vol12.sp.8.2025Keywords:
4-Sulfocalix[4]arene, Atomic Force Microscope, Spin-Coating Method, Thickness Measurement, Topographical AnalysisAbstract
This study aims to characterise 4-Sulfocalix[4]arene (SC[4]) thin films using Atomic Force Microscopy (AFM) in analysing its thickness and surface morphology. Quartz substrates were cleaned using ultrasonic cleaning with an Elmasonic P70H. SC[4] thin films were then fabricated on a quartz surface using WS-400BZ-6NPP/A1/AR1 model spin coater from Laurel Technologies, producing thin films with 5, 10, 15, and 20 layers. Each thin film was characterised in the tapping mode of the AFM Park System NX-10. Scans were conducted on different film areas of different scales, and the data were analysed using XEI Data Processing and Analysis Software. Height measurements obtained via Line Profile analysis revealed a progressive increase in film thickness with the additional layers, demonstrating a significant linear relationship. AFM images showed precise 2D and 3D surface topography, indicating that the thin films became uneven and non-uniform with the addition of more layers. The findings demonstrated a linear relationship between film thickness and the number of layers, measuring 58.175 nm for 5 layers, 77.626 nm for 10 layers, 84.608 nm for 15 layers, and 94.806 nm for 20 layers. The surface roughness of thin films was also determined, and it is significantly influenced by the number of layers, as illustrated by the root mean square roughness (Rq). The values were found to be 26.512 for 5 layers, 29.777 for 10 layers, 30.177 for 15 layers, and 31.093 for 20 layers, respectively. This increase in surface irregularity indicates that the deposition process leads to progressively irregular surfaces with an additional number of layers, as proven by equivalent studies on thin films. This study emphasised the effectiveness of AFM in thin film research by demonstrating its precision in measuring thickness and analysing surface topography.
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References
[1] L. Garcia-Rio, N. Basílio, and V. Francisco, ‘Counterion effect on sulfonatocalix[n]arene recognition’, Pure Appl. Chem., vol. 92, no. 1, pp. 25–37, Jan. 2020, doi: 10.1515/pac-2019-0305.
[2] S. Shinkai, ‘Calixarenes-The Third Generation of Supramolecules’, Tetrahedron, vol. 49, no. 40, pp. 8933–8968, 1993.
[3] A. de Fatima, S. Fernandes, and A. Sabino, ‘Calixarenes as New Platforms for Drug Design’, Curr. Drug Discov. Technol., vol. 6, no. 2, 2009, doi: 10.2174/157016309788488302.
[4] H. Li and Y. W. Yang, ‘Gold nanoparticles functionalized with supramolecular macrocycles’, Chin. Chem. Lett., vol. 24, no. 7, pp. 545–552, 2013, doi: 10.1016/j.cclet.2013.04.014.
[5] V. Bohmer, ‘Calixarenes, Macrocycles with (Almost) Unlimited Possibilities’, Angew. Chem. Int. Ed. Engl., vol. 34, no. 7, pp. 713–745, 1995.
[6] S. Moffa et al., ‘Synthesis, characterization, and computational study of aggregates from amphiphilic calix[6]arenes. Effect of encapsulation on degradation kinetics of curcumin’, J. Mol. Liq., vol. 368, p. 120731, Dec. 2022, doi: 10.1016/j.molliq.2022.120731.
[7] E. Español and M. Villamil, ‘Calixarenes: Generalities and Their Role in Improving the Solubility, Biocompatibility, Stability, Bioavailability, Detection, and Transport of Biomolecules’, Biomolecules, vol. 9, no. 3, p. 90, Mar. 2019, doi: 10.3390/biom9030090.
[8] R. Ludwig, ‘Calixarenes in analytical and separation chemistry’, Fresenius J. Anal. Chem., vol. 367, no. 2, pp. 103–128, May 2000, doi: 10.1007/s002160051611.
[9] G. Sachdeva et al., ‘Calix[n]arenes and its derivatives as organocatalysts’, Coord. Chem. Rev., vol. 472, p. 214791, Dec. 2022, doi: 10.1016/j.ccr.2022.214791.
[10] C. Jin Mei and S. Ainliah Alang Ahmad, ‘A review on the determination heavy metals ions using calixarene-based electrochemical sensors’, Arab. J. Chem., vol. 14, no. 9, Sep. 2021, doi: 10.1016/j.arabjc.2021.103303.
[11] A. Gorbunov et al., ‘Selective azide-alkyne cycloaddition reactions of azidoalkylated calixarenes’, Org. Chem. Front., vol. 7, no. 17, pp. 2432–2441, Sep. 2020, doi: 10.1039/d0qo00650e.
[12] A. Awasthi, P. Jadhao, and K. Kumari, ‘Clay nano-adsorbent: structures, applications and mechanism for water treatment’, SN Appl. Sci., vol. 1, no. 9, Sep. 2019, doi: 10.1007/s42452-019-0858-9.
[13] R. Sivashankar, A. B. Sathya, K. Vasantharaj, and V. Sivasubramanian, ‘Magnetic composite an environmental super adsorbent for dye sequestration - A review’, Environ. Nanotechnol. Monit. Manag., vol. 1–2, pp. 36–49, Nov. 2014, doi: 10.1016/j.enmm.2014.06.001.
[14] V. Montes-García, J. Pérez-Juste, I. Pastoriza-Santos, and L. M. Liz-Marzán, ‘Metal nanoparticles and supramolecular macrocycles: A tale of synergy’, Chem. - Eur. J., vol. 20, no. 35, pp. 10874–10883, Aug. 2014, doi: 10.1002/chem.201403107.
[15] G. Mcmahon, S. O’malley, K. Nolan, and D. Diamond, ‘Important calixarene derivatives-their synthesis and applications’, Arkivoc, vol. 2003, no. 7, pp. 23–31, Apr. 2003, doi: http://dx.doi.org/10.3998/ark.5550190.0004.704.
[16] N. I. Ruslan, D. C. K. Lim, S. A. Alang Ahmad, S. F. N. Abdul Aziz, F. L. Supian, and N. A. Yusof, ‘Ultrasensitive electrochemical detection of metal ions using dicarboethoxycalixarene-based sensor’, J. Electroanal. Chem., vol. 799, pp. 497–504, Aug. 2017, doi: 10.1016/j.jelechem.2017.06.038.
[17] A. Shah, ‘A Novel Electrochemical Nanosensor for the Simultaneous Sensing of Two Toxic Food Dyes’, ACS Omega, vol. 5, no. 11, pp. 6187–6193, Mar. 2020, doi: 10.1021/acsomega.0c00354.
[18] S. Abubakar, T. Skorjanc, D. Shetty, and A. Trabolsi, ‘Porous Polycalix[n]arenes as Environmental Pollutant Removers’, ACS Appl. Mater. Interfaces, vol. 13, no. 13, pp. 14802–14815, Apr. 2021, doi: 10.1021/acsami.0c23074.
[19] S. Pilato et al., ‘Calixarene-based artificial ionophores for chloride transport across natural liposomal bilayer: Synthesis, structure-function relationships, and computational study’, Biochim. Biophys. Acta BBA - Biomembr., vol. 1863, no. 10, p. 183667, Oct. 2021, doi: 10.1016/j.bbamem.2021.183667.
[20] T. Nishikubo, A. Kameyama, and H. Kudo, ‘Novel High Performance Materials. Calixarene Derivatives Containing Protective Groups and Polymerizable Groups for Photolithography, and Calixarene Derivatives Containing Active Ester Groups for Thermal Curing of Epoxy Resins’, Polym. J., vol. 35, no. 3, pp. 213–229, 2003, doi: http://dx.doi.org/10.1295/polymj.35.213.
[21] X. Tian et al., ‘4-Sulfocalix[4]arene/Cucurbit[7]uril-Based Supramolecular Assemblies through the Outer Surface Interactions of Cucurbit[ n]uril’, ACS Omega, vol. 3, no. 6, pp. 6665–6672, Jun. 2018, doi: 10.1021/acsomega.8b00829.
[22] X. Y. Hu, S. Peng, D. S. Guo, F. Ding, and Y. Liu, ‘Molecular recognition of amphiphilic p -sulfonatocalix[4]arene with organic ammoniums’, Supramol. Chem., vol. 27, pp. 336–345, 2015, doi: 10.1080/10610278.2014.967242.
[23] S. Shinkai, S. Mori, T. Tsubaki, T. Sone, and O. Manabe, ‘New water-soluble host molecules derived from calix[6]arene’, Tetrahedron Lett., vol. 25, no. 46, 1984, doi: 10.1016/S0040-4039(01)81592-6.
[24] P. Shivappa Adarakatti, C. W. Foster, C. E. Banks, A. K. N. S., and P. Malingappa, ‘Calixarene bulk modified screen-printed electrodes (SPCCEs) as a one-shot disposable sensor for the simultaneous detection of lead(II), copper(II) and mercury(II) ions: Application to environmental samples’, Sens. Actuators Phys., vol. 267, pp. 517–525, Nov. 2017, doi: 10.1016/j.sna.2017.10.059.
[25] Ö. Mermer, S. Okur, F. Sümer, C. Özbek, S. Sayın, and M. Yılmaz, ‘Gas Sensing Properties of Carbon Nanotubes Modified with Calixarene Molecules Measured by QCM Techniques’, Acta Phys. Pol. A, vol. 121, no. 1, pp. 240–242, Jan. 2012, doi: 10.12693/APhysPolA.121.240.
[26] X. Liang, D. M. King, and A. W. Weimer, ‘Ceramic ultra-thin coatings using atomic layer deposition’, in Ceramic Nanocomposites, Rajat Banerjee and Indranil Manna, Eds., Elsevier, 2013, pp. 257–283. doi: 10.1533/9780857093493.2.257.
[27] R. Asmatulu and W. S. Khan, ‘Characterization of electrospun nanofibers’, in Synthesis and Applications of Electrospun Nanofibers, Elsevier, 2019, pp. 257–281. doi: 10.1016/B978-0-12-813914-1.00013-4.
[28] M. F. Erinosho and E. T. Akinlabi, ‘Estimation of Surface Topography and Wear Loss of Laser Metal‐Deposited Ti6Al4V and Cu’, Adv. Eng. Mater., vol. 18, no. 8, pp. 1396–1405, Aug. 2016, doi: 10.1002/adem.201600062.
[29] Z.-N. Fang, B. Yang, M.-G. Chen, C.-H. Zhang, J.-P. Xie, and G.-X. Ye, ‘Growth and morphology of ultra-thin Al films on liquid substrates studied by atomic force microscopy’, Thin Solid Films, vol. 517, no. 11, pp. 3408–3411, Apr. 2009, doi: 10.1016/j.tsf.2009.01.017.
[30] M. Kwoka, L. Ottaviano, and J. Szuber, ‘AFM study of the surface morphology of L-CVD SnO2 thin films’, Thin Solid Films, vol. 515, no. 23, pp. 8328–8331, Sep. 2007, doi: 10.1016/j.tsf.2007.03.035.
[31] J. Xie, X. Lu, Y. Zhu, C. Liu, N. Bao, and X. Feng, ‘Atomic force microscopy (AFM) study on potassium hexatitanate whisker (K2O·6TiO2)’, J. Mater. Sci., vol. 38, no. 17, pp. 3641–3646, Sep. 2003, doi: 10.1023/A:1025685516330.
[32] M. Cremona et al., ‘Grain size distribution analysis in polycrystalline LiF thin films by mathematical morphology techniques on AFM images and X‐ray diffraction data’, J. Microsc., vol. 197, no. 3, pp. 260–267, Mar. 2000, doi: 10.1046/j.1365-2818.2000.00661.x.
[33] Y. Gong, S. T. Misture, P. Gao, and N. P. Mellott, ‘Surface Roughness Measurements Using Power Spectrum Density Analysis with Enhanced Spatial Correlation Length’, J. Phys. Chem. C, vol. 120, no. 39, pp. 22358–22364, Oct. 2016, doi: 10.1021/acs.jpcc.6b06635.
[34] T. Ando, T. Uchihashi, and S. Scheuring, ‘Filming biomolecular processes by high-speed atomic force microscopy’, Chem. Rev., vol. 114, no. 6, pp. 3120–3188, Mar. 2014, doi: 10.1021/cr4003837.
[35] G. Binnig, C. F. Quate’ ’, E. L. Gi, and C. Gerber, ‘Atomic Force Microscope’, Phys. Rev. Lett., vol. 56, no. 9, pp. 930–933, Mar. 1996, doi: 10.1103/PhysRevLett.56.930.
[36] P. S. Liu and G. F. Chen, ‘Characterization Methods’, in Porous Materials, Elsevier, 2014, pp. 411–492. doi: 10.1016/B978-0-12-407788-1.00009-5.
[37] P. J. D. Whiteside, J. A. Chininis, and H. K. Hunt, ‘Techniques and challenges for characterizing metal thin films with applications in photonics’, Coatings, vol. 6, no. 3, pp. 1–26, Sep. 2016, doi: 10.3390/coatings6030035.
[38] A. Abdelbary and L. Chang, ‘Properties and characteristics of tribo-surfaces’, in Principles of Engineering Tribology, Elsevier, 2023, pp. 33–75. doi: 10.1016/B978-0-323-99115-5.00009-8.
[39] K. Munir, A. Biesiekierski, C. Wen, and Y. Li, ‘Surface modifications of metallic biomaterials’, in Metallic Biomaterials Processing and Medical Device Manufacturing, C. Wen, Ed., Elsevier, 2020, pp. 387–424. doi: 10.1016/B978-0-08-102965-7.00012-6.
[40] E. Thormann, ‘Surface forces between rough and topographically structured interfaces’, Curr. Opin. Colloid Interface Sci., vol. 27, pp. 18–24, Feb. 2017, doi: 10.1016/j.cocis.2016.09.011.
[41] W. Chrzanowski and F. Dehghani, ‘Standardised chemical analysis and testing of biomaterials’, in Standardisation in Cell and Tissue Engineering, V. Salih, Ed., Elsevier, 2013, pp. 166–197a. doi: 10.1533/9780857098726.2.166.
[42] Q. Xin, ‘Friction and lubrication in diesel engine system design’, in Diesel Engine System Design, Elsevier, 2013, pp. 651–758. doi: 10.1533/9780857090836.3.651.
[43] A. Badarneh, J. J. E. Choi, K. Lyons, J. N. Waddell, and K. C. Li, ‘Wear Behaviour of Monolithic Zirconia Against Human Enamel – A Literature Review’, Biotribology, vol. 32, p. 100224, Sep. 2022, doi: 10.1016/j.biotri.2022.100224.
[44] S. Gowri et al., ‘Atomic force microscopy technique for corrosion measurement’, in Electrochemical and Analytical Techniques for Sustainable Corrosion Monitoring, A. Jeenat, C. Verma, and C. M. Hussain, Eds., Elsevier, 2023, pp. 121–140. doi: 10.1016/B978-0-443-15783-7.00001-3.
[45] Q. Liu, Y. Fu, Z. Qin, Y. Wang, S. Zhang, and M. Ran, ‘Progress in the applications of atomic force microscope (AFM) for mineralogical research’, Micron, vol. 170, p. 103460, Jul. 2023, doi: 10.1016/j.micron.2023.103460.
[46] J. A. Ogilvy, ‘A Model for the Ultrasonic Inspection of Rough Defects’, in Non-Destructive Testing, vol. 1, J. Boogaard and G. M. van Dijk, Eds., Amsterdam: Elsevier, 1989, pp. 830–835. doi: 10.1016/B978-0-444-87450-4.50175-2.
[47] J. Qu, ‘Thermomechanical Reliability of Microelectronic Packaging’, in Comprehensive Structural Integrity, vol. 8, I. Milne, R. Ritchie, and B. Karihaloo, Eds., Elsevier, 2003, pp. 219–239. doi: 10.1016/B0-08-043749-4/08040-X.
[48] F. Quartinello, G. M. Guebitz, and D. Ribitsch, ‘Surface functionalization of polyester’, in Methods in Enzymology, vol. 627, N. Bruns and K. Loos, Eds., Academic Press, 2019, pp. 339–360. doi: 10.1016/bs.mie.2019.08.007.
[49] A. S. Razali, F. L. Supian, S. Abu Bakar, T. H. Richardson, and N. A. Azahari, ‘The properties of carbon nanotube on novel calixarene thin film’, Int. J. Nanoelectron. Mater., vol. 8, pp. 39–45, 2015.
[50] C. K. D. Lim and F. L. Supian, ‘Calix[4]arene and calix[8]arene Langmuir films: Surface studies, optical and structural characterizations’, Int. J. Innov. Technol. Explor. Eng., vol. 8, no. 8, 2019.
[51] T. Fujimoto, K. Takeda, and T. Nonaka, ‘Airborne Molecular Contamination: Contamination on Substrates and the Environment in Semiconductors and Other Industries’, in Developments in Surface Contamination and Cleaning, vol. 1, William Andrew Publishing, 2008, pp. 197–329. doi: 10.1016/B978-0-323-29960-2.00007-1.
[52] J. X. J. Zhang and K. Hoshino, ‘Fundamentals of nano/microfabrication and scale effect’, in Molecular Sensors and Nanodevices, Academic Press, 2018, pp. 43–111. doi: 10.1016/b978-0-12-814862-4.00002-8.
[53] B. S. Yilbas, A. Al-Sharafi, and H. Ali, ‘Surfaces for Self-Cleaning’, in Self-Cleaning of Surfaces and Water Droplet Mobility, Elsevier, 2019, pp. 45–98. doi: 10.1016/b978-0-12-814776-4.00003-3.
[54] A. Mishra, N. Bhatt, and A. K. Bajpai, ‘Nanostructured superhydrophobic coatings for solar panel applications’, in Nanomaterials-Based Coatings: Fundamentals and Applications, Elsevier, 2019, pp. 397–424. doi: 10.1016/B978-0-12-815884-5.00012-0.
[55] A. Boudrioua, M. Chakaroun, and A. Fischer, ‘Organic Light-emitting Diodes’, in An introduction to organic lasers, ISTE Press - Elsevier, 2017, pp. 49–93.
[56] F. L. Supian, D. C. K. Lim, and A. S. Razali, ‘Conductivity comparison of calix[8]arene-MWCNTs through spin coating technique’, Sains Malays., vol. 46, no. 1, pp. 91–96, Jan. 2017, doi: 10.17576/jsm-2017-4601-12.
[57] A. K. Hassan et al., ‘Thin films of calix-4-resorcinarene deposited by spin coating and Langmuir-Blodgett techniques: determination of film parameters by surface plasmon resonance’, Mater. Sci. Eng. C, vol. 8–9, pp. 251–255, Dec. 1999.
[58] S. Romano et al., ‘Optical Biosensors Based on Photonic Crystals Supporting Bound States in the Continuum’, Materials, vol. 11, no. 4, p. 526, Mar. 2018, doi: 10.3390/ma11040526.
[59] I. Kubicova, J. Skriniarova, D. Pudis, L. Suslik, and M. Vesely, ‘Non-Contact NSOM Lithography for 2D Photonic Structure Fabrication’, Phys. Procedia, vol. 32, pp. 113–116, 2012, doi: 10.1016/j.phpro.2012.03.527.
[60] A. Bakry and S. M. Elmesallamy, ‘Sulfonated polypropylene microparticles from waste as adsorbents for methylene blue: Kinetic, equilibrium, and thermodynamic studies’, Sep. Sci. Technol., vol. 57, no. 15, pp. 2374–2392, Oct. 2022, doi: 10.1080/01496395.2022.2064874.
[61] N. H. Thang, T. B. Chien, and D. X. Cuong, ‘Polymer-Based Hydrogels Applied in Drug Delivery: An Overview’, Gels, vol. 9, no. 7, p. 523, Jun. 2023, doi: 10.3390/gels9070523.
[62] W. Zhao et al., ‘Sulfonate-grafted conjugated microporous polymers for fast removal of cationic dyes from water’, Chem. Eng. J., vol. 391, p. 123591, Jul. 2020, doi: 10.1016/j.cej.2019.123591.
[63] Y. Yan, P. Zhou, S.-X. Zhang, X.-G. Guo, and D.-M. Guo, ‘Effect of substrate curvature on thickness distribution of polydimethylsiloxane thin film in spin coating process’, Chin. Phys. B, vol. 27, no. 6, pp. 1–9, Jun. 2018, doi: 10.1088/1674-1056/27/6/068104.
[64] Y. Yan, J. Li, Q. Liu, and P. Zhou, ‘Evaporation Effect on Thickness Distribution for Spin-Coated Films on Rectangular and Circular Substrates’, Coatings, vol. 11, no. 11, pp. 1–16, Oct. 2021, doi: 10.3390/coatings11111322.
[65] K. V. Kumar et al., ‘Characterization of the adsorption site energies and heterogeneous surfaces of porous materials’, J. Mater. Chem. A, vol. 7, no. 17, pp. 10104–10137, 2019, doi: 10.1039/C9TA00287A.
[66] X. Ou, X. Liu, W. Liu, W. Rong, J. Li, and Z. Lin, ‘Surface defects enhance the adsorption affinity and selectivity of Mg(OH) 2 towards As( v ) and Cr( vi ) oxyanions: a combined theoretical and experimental study’, Environ. Sci. Nano, vol. 5, no. 11, pp. 2570–2578, 2018, doi: 10.1039/C8EN00654G.
[67] M. Takayanagi, N. Fujiwara, R. Seki, M. Sato, and Y. Okuno, ‘Amorphous SiO2 Surface Irregularities and their Influence on Liquid Molecule Adsorption by Molecular Dynamics Analysis’, ECS J. Solid State Sci. Technol., vol. 12, no. 8, p. 083003, Aug. 2023, doi: 10.1149/2162-8777/acec0e.
[68] I. Isakov et al., ‘Quantum Confinement and Thickness‐Dependent Electron Transport in Solution‐Processed In 2 O 3 Transistors’, Adv. Electron. Mater., vol. 6, no. 11, pp. 1–7, Nov. 2020, doi: 10.1002/aelm.202000682.
[69] F. Güzelçimen et al., ‘The effect of thickness on surface structure of rf sputtered TiO2 thin films by XPS, SEM/EDS, AFM and SAM’, Vacuum, vol. 182, pp. 1–14, Dec. 2020, doi: 10.1016/j.vacuum.2020.109766.
[70] A. Aziz, A. N. Afaah, N. A. M. Asib, R. Mohamed, M. Rusop, and Z. Khusaimi, ‘Surface Morphology Studies on Different Layers of PMMA Spin Coated onto Substrate Prepared by Sol-Gel Spin-Coating Method’, Adv. Mater. Res., vol. 1109, pp. 608–612, Jun. 2015, doi: 10.4028/www.scientific.net/AMR.1109.608.
[71] M. A. Basar, S. Debnath, and A. B. Md Ismail, ‘An experimental study on the effect of film layer and annealing on morphology by AFM of tin dioxide thin film prepared by spin-coating method’, Mater. Res. Express, vol. 1, no. 2, p. 026402, May 2014, doi: 10.1088/2053-1591/1/2/026402.
[72] T. Kohoutek et al., ‘Surface morphology of spin-coated As–S–Se chalcogenide thin films’, J. Non-Cryst. Solids, vol. 353, no. 13–15, pp. 1437–1440, May 2007, doi: 10.1016/j.jnoncrysol.2006.10.068.
[73] A. R. Arul, H. B. Ramalingam, R. Balamurugan, and R. Venckatesh, ‘Optically rough TiO2 thin film surface study by laser speckle photography’, in Materials Today: Proceedings, 2023, pp. 1381–1384. doi: 10.1016/j.matpr.2023.05.535.
[74] A. Beena Unni, R. Winkler, D. M. Duarte, K. Chat, and K. Adrjanowicz, ‘Influence of Surface Roughness on the Dynamics and Crystallization of Vapor-Deposited Thin Films’, J. Phys. Chem. B, vol. 126, no. 40, pp. 8072–8079, Oct. 2022, doi: 10.1021/acs.jpcb.2c04541.
[75] K. Kalantari, B. Saleh, and T. J. Webster, ‘Applications of Thin Films in Metallic Implants’, in Materials for Devices, Boca Raton: CRC Press, 2022, pp. 271–304. doi: 10.1201/9781003141358-10.
[76] M. Mozetič, ‘Surface Modification to Improve Properties of Materials’, Materials, vol. 12, no. 3, p. 441, Jan. 2019, doi: 10.3390/ma12030441.
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