Silicon Fiber Optic Coating with Zinc Oxide Nanoparticles Characterized by AFM ^†

Abstract

This paper presents the preparation and characterization of single-mode optical fibers coated with zinc oxide (ZnO) nanoparticles using the immersion technique. The study was carried out in three stages: the first consisted of pretreating the fiber by means of controlled immersion in HCl and H₂SO₄ solutions and exposure in a muffle furnace; the second involved the growth and deposition of ZnO nanoparticles synthesized in a laboratory; and the third was characterization by means of atomic force microscopy (AFM). In this last stage, we obtained through AFM that Sample 1, considered optimized, presented high particle density (9.203 particles/µm²), an RMS roughness (Rq) of 2.98 nm, and average roughness (Ra) of 1.82 nm, as well as an average height of 1.117 nm. These parameters reflect a uniform and stable surface, desirable conditions for applications in the development of high-sensitivity optical sensors and biosensors.

1. Introduction

Optical sensors have established themselves as highly sensitive and specific tools for detecting environmental pollutants [1], clinical biomarkers [2], and gases in various contexts [2,3]. Their ability to provide rapid, non-invasive measurements has driven their implementation in both environmental monitoring [4,5] and biomedical applications [6]. However, the effectiveness of these sensors depends mainly on the interaction between light and the surface of the device, which has led to the incorporation of functional coatings at the nanometric scale that modify the optical and chemical properties of the fiber, increasing its sensitivity and range of application [7,8].

Several studies have explored the use of nanomaterials such as germanium and silver [9], gold [10], graphene [11], combinations of silver and platinum [12], and silver with zirconia [13] to improve the response of optical devices. These studies have shown that the choice of material and deposition technique plays a crucial role in directly influencing the uniformity of the coating and the reproducibility of the sensor. Understanding and optimizing these factors is of utmost importance in our research. In this context, zinc oxide (ZnO) has attracted considerable interest due to its semiconductor properties, chemical stability, low cost, and compatibility with simple deposition methods, such as dip-coating, which allow the fiber surface to be modified in a controlled and uniform manner [6,13,14].

Parameters such as stability and sensitivity are of great importance for optical sensors, as they largely depend on the coating applied to the optical fiber [15,16]. In [17], a more detailed study of the effects of coatings can be found. Pacheco-Chacón (2021) demonstrated a temperature sensor before and after aluminum coating, which produced an increase in temperature sensitivity from approximately 35 to 120 pm/°C [18].

In this study, the term nanoparticles (NPs) is used to refer to ZnO particles with an average size of less than 100 nm, characterized by atomic force microscopy (AFM), which justifies their designation as nanometric. The incorporation of these nanoparticles not only improves the interaction of light with the external environment but also increases detection sensitivity, a critical aspect for the development of advanced optical sensors [19,20,21].

This study focuses on the development of a protocol for coating single-mode silicon fiber optics (SMF-28) by immersion, optimizing the chemical pretreatment of the surface and the deposition of ZnO nanoparticles to achieve a homogeneous and stable coating. The standardization of this procedure seeks to lay the foundations for the manufacturing of low-cost, high-sensitivity optical sensors with potential applications in both environmental monitoring and the biomedical field. In addition, this study contributes to establishing methodological criteria for the selection of immersion times, acid treatment, and nanoparticle deposition, which are fundamental elements for obtaining reproducible coatings with improved optical properties.

2. Materials and Methods

The experimental procedure was carried out using a single-mode silicon optical fiber (SMF-28, Thorlabs, Newton, NJ, USA) to modify its surface by coating it with zinc oxide (ZnO nanoparticles, previously synthesized in the laboratory) using the dip-coating technique. The protocol was divided into three stages, namely fiber preparation, ZnO solution preparation, and coating application, followed by characterization using atomic force microscopy (AFM) (Nanoscope, Bruker Corporation, Billerica, MA, USA).

To prepare the optical fiber, it was cut into segments of approximately 10 cm and subjected to a three-step chemical pretreatment. First, an initial wash was performed by immersion in distilled water and 99.9% absolute ethanol (v/v) (generic laboratory supplier, Mexico) for 5 min to remove surface organic contaminants. Subsequently, as shown in Figure 1, the fiber was immersed in concentrated sulfuric acid (H₂SO₄, 95–98%, Omnichen ACS, Puebla, México) for 40 min at room temperature, with occasional stirring, allowing the protective polymer coating to be removed and exposing the silica core of the optical fiber, as shown in Figure 1a. After this step, a second wash was performed with 99.9% ethanol and distilled water, as shown in Figure 1b. The fiber was then immersed in concentrated hydrochloric acid (HCl, 37%, Omnichen ACS, Puebla, México) for 17 h, generating a controlled attack that increased the surface roughness and promoted the adhesion of the nanoparticles. As shown in Figure 1c, it was finally rinsed with abundant distilled water and 99.9% ethanol to remove acid residues and left to dry at room temperature. Immersion times were optimized experimentally to achieve the desired roughness without compromising fiber integrity. The acids used can be decanted and reused, following established safety and chemical waste management protocols.

The ZnO solution was prepared in a 1:1 (v/v) mixture of 99.9% absolute ethanol and distilled water, using 5 mL of each solvent. In this solution, 0.05 g 0 were dispersed, which were previously synthesized in a laboratory and have an average size of less than 100 nm, as verified by AFM, shown in Figure 2a. The suspension was sonicated in an ultrasonic cleaner (Branson 1510, Branson Ultrasonics Corporation, Danbury, CT, USA) for 5 min; the device only allows for control of the sonication time, not the power, to ensure homogeneous dispersion and minimize the formation of aggregates, as shown in Figure 2b. Subsequently, the treated fibers were placed in a single immersion in a porcelain capsule and subjected to heat treatment in a muffle (Thermolyne Small Benchtop Muffle Furnace, Thermo Fisher Scientific, Dubuque, IA, USA) to 500 °C for 10 min as a drying procedure (Figure 2c), which allowed for partial sintering of the nanoparticles on the fiber surface, ensuring adhesion without affecting its structure.

The coating was characterized by using atomic force microscopy (AFM) with Innova AFM equipment (Bruker) (Nanoscope, Bruker Corporation, Billerica, MA, USA).

The coated fibers were fixed to the sample holder with double-sided conductive tape and analyzed in tapping mode. The tip used was an RTESP-300 AFM probe (Bruker Nano Surfaces, USA) made of antimony-doped silicon (0.01–0.025 Ω·cm), with a cantilever 125 μm long, 40 μm wide, and 3.4 μm thick, a resonance frequency of 300 kHz, and a spring constant of 40 N/m. The back of the tip had a reflective aluminum coating, while the front had no coating.

Height and phase images were obtained over areas of 250 × 250 nm, using a scanning speed ranging from 0.5 to 1.5 kHz to obtain optimal images. Each sample was analyzed in three replicates to ensure reproducibility, and the images were used to calculate the average roughness (Ra), RMS roughness (Rq), minimum, maximum, and average height, and particle distribution on the fiber surface.

3. Results

This study was conducted in two stages, in which changes in the roughness of the optical fiber were evaluated, allowing progress to be made towards the controlled deposition of ZnO nanoparticles using the immersion technique. The samples were characterized using atomic force microscopy (AFM, Innova Bruker), which allowed the uniformity and roughness of the coating to be quantified and the three samples studied to be systematically compared (

Table 1. Roughness parameters and characteristics of optical fiber samples coated with ZnO nanoparticles.

Parameter	Sample 1	Sample 2	Sample 3
Total count	416	350	280
Density (/µm2)	9.203	6.500	4.120
Image Rq (nm)	2.98	6.20	42.2
Image Ra (nm)	1.82	2.30	19.4
Image Rmax (nm)	53.9	118	586
Average height (nm)	1.117	3.450	5.980
Minimum height (nm)	0.596	0.100	0.500
Maximum height (nm)	26.802	35.200	42.500
Standard deviation height (nm)	1.630	4.220	6.780
Hydrochloric acid immersion time (h)	17	12	8
Sulfuric acid immersion time (min)	40	40	40
Muffle time (min)	10	7	5

).

These include particle count, density, RMS roughness (Rq), average arithmetic roughness (Ra), minimum, maximum, and average height, standard deviation of height, and immersion times in HCl and H₂SO₄, as well as the duration of heat treatment in a muffle furnace.

Sample 1, considered the most representative, had a total count of 416 particles with a density of 9.203 particles/µm², an average height of 1.117 nm, and a standard deviation of 1.630 nm, indicating a highly homogeneous surface. The RMS roughness (Rq) and arithmetic mean (Ra) were 2.98 nm and 1.82 nm, respectively, showing that acid pretreatment with HCl for 17 h and H₂SO₄ for 40 min, together with exposure in a muffle furnace for 10 min, allowed for controlled nucleation of the nanoparticles and uniform adhesion. These conditions minimized the formation of aggregates and pronounced peaks, increasing the potential of the optical fiber for applications in high-sensitivity sensors.

Figure 3 illustrates the evolution of the optical fiber, captured using a professional trinocular metallurgical optical microscope (AmScope T800M, designed/shipped from Irvine, CA, USA; manufactured in China) with an optical zoom of 20×. Figure 3a shows the uncoated surface, while Figure 3b, shows the surface after acid treatment, which exhibits an increase in surface roughness that favors the adhesion of nanoparticles.

Figure 4 presents AFM images of Sample 1, where the height topography and voltage measurement show a homogeneous distribution of particles with an average size of 30 nm, confirming the effectiveness of the deposition protocol.

Sample 2 had a lower density (6.500 particles/µm²) and an increase in average height (3.450 nm) and standard deviation (4.220 nm), indicating less uniform surfaces with greater roughness. The Rq (6.20 nm) and Ra (2.30 nm) values show localized irregularities, resulting from shorter immersion times in HCl and H₂SO₄ (12 h and 40 min, respectively) and shorter exposure in the muffle furnace (7 min). These conditions resulted in a less homogeneous coating and highlight the sensitivity of the process to variations in treatment times.

Finally, Sample 3 showed the least favorable results, with a density of 4.120 particles/µm², an average height of 5.980 nm, and a standard deviation of 6.780 nm, reflecting a highly irregular surface with significant aggregates. The roughness Rq (42.2 nm) and Ra (19.4 nm) confirm the formation of pronounced peaks, resulting from reduced immersion times in HCl (8 h) and muffle exposure (5 min), which did not allow for the adequate nucleation or uniform distribution of ZnO nanoparticles.

In summary, the results indicate that Sample 1 achieved optimal coating conditions, demonstrating that the combination of immersion times in acids and muffle exposure is decisive in obtaining uniform surfaces with low roughness. Figure 4 of AFM and the comparison with Figure 3 clearly show how the roughness generated during pretreatment favors the adhesion and homogeneous distribution of nanoparticles, which increases the potential of the optical fiber in the development of high-sensitivity and high-precision optical sensors, underlining the urgent need for further research in this area.

4. Discussion

The results obtained show that the combination of acid pretreatment and controlled exposure in a muffle furnace significantly influences the roughness and uniformity of the ZnO nanoparticle coating on single-mode optical fibers. Sample 1, with longer immersion times in H₂SO₄ and adequate muffle exposure, presented the highest particle density and lowest roughness in the coating, indicating that the prepared surface facilitated homogeneous nucleation and efficient adhesion of the nanoparticles. This behavior is consistent with previous reports where a controlled increase in surface roughness favors the interaction between light and the external medium, increasing the sensitivity of optical sensors [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22].

On the other hand, Sample 2 and Sample 3, with shorter immersion and exposure times, exhibited lower particle densities and higher roughness in the coating, indicating less controlled nucleation and the formation of aggregates. This underscores the sensitivity of the coating process to variations in chemical and thermal treatment times but also underscores the potential for experimental optimization of these parameters, empowering researchers to enhance sensor performance.

The development of homogeneous coatings by immersion is a versatile method that can be adapted to different types of nanomaterials. It allows for the uniform deposition of nanoparticles on the fiber, is reproducible, and facilitates the control of parameters such as particle density and size. Surface roughness parameters (Ra and Rq) vary significantly depending on the deposition method and the intended application of the material.

Table 2. Comparison of surface roughness values (Ra and Rq) reported in the literature.

Coating Method	Sample Type	Sample or Condition	Ra (nm)	Rq (nm)	Ref
Magnetron sputtering	ZnO thin film on glass	300 °C	3.57	4.49	[23]
Chemical deposition	ZnO nanostructures	5 min H2	3.12	3.97	[24]
Sol–gel (ZnO:Mg)	ZnO:Mg films	Pure ZnO	4.04	5.16	[25]
RF sputtering	Irradiated ZnO films	Non-irradiated ZnO	18.65	22.94	[26]
Immersion (HCl acid + H2SO4 + muffle furnace)	Optical fiber coated with ZnO	Sample 1	1.82	2.98	This work

presents values that favor optical transmission. Kaim et al. [23] reported Ra = 3.57 nm and Rq = 4.49 nm for films obtained by the sputtering technique, while Lee et al. [24] achieved Ra = 3.12 nm and Rq = 3.97 nm using nanoporous structures. Sarıca et al. [25] observed an increase in roughness in Mg-coated samples (Ra = 4.04 nm; Rq = 5.16 nm), and Duinong et al. [26] obtained significantly higher values (Ra = 18.65 nm; Rq = 22.94 nm) after gamma irradiation. Each ZnO based study reports materials prepared according to the methodology and specific requirements of their intended application.

Smooth surfaces enhance light absorption and reduce scattering, whereas excessive roughness can lead to strong light dispersion. In this study, a ZnO coating was deposited on an optical fiber surface via chemical immersion, yielding Ra = 1.82 nm and Rq = 2.98 nm, which are considered indicative of a smooth surface. The resulting coating may be suitable for optical sensors, such as surface plasmon resonance (SPR) devices [27], as it can enhance the surface plasmonic effect and improve sensor efficiency.

Furthermore, the results suggest that the optical properties of the fiber, such as the refractive index, reflectivity, and sensitivity to changes in the environment, could be improved by ZnO coatings. This discovery not only validates our research but also opens up the thrilling possibility of applying this type of coating in optical sensors and biosensors, where controlled interaction between light and functionalized surfaces is crucial. The potential for future applications is fascinating.

5. Conclusions

Optimizing acid pretreatment times and muffle exposure times was crucial to obtaining homogeneous, low-roughness ZnO coatings on single-mode optical fibers. Sample 1 had the highest particle density and most uniform surface, indicating controlled nucleation and efficient adhesion of the nanoparticles. These findings demonstrate that the combination of chemical and thermal treatments used improves the optical properties of the fiber, increasing its potential for applications in high-sensitivity, high-precision optical sensors and biosensors.