Introduction

The industrial use of nanomaterials (NMs) and nanoparticles (NPs) in consumer products has proliferated, as found in electronic components [1], sports equipment [2], textile products [3], cosmetics [4], and biomedical applications [5–8]. Upon disposal of these products, these NMs often afford higher environmental mobility and will inevitably lead to a wide range of human exposure to aluminum oxide, carbon, copper oxide, gold, hematite, magnetite, manganese oxide [9], silver [10], titanium oxide [11], and polymer composite nanomaterials. The toxicity of NMs arises from a number of biophysicochemical factors, including their ability to penetrate biological barriers, tissues, and cells. Their large surface area-to-mass ratio increases oxidative stress, resulting in undesirable interactions with biological macromolecules. Alumina and silica are among those inorganic NPs most often used in the industry even though their toxicity is controversial [12, 13]. Colloidal silica nanoparticles are more and more often used in various biomedical applications [14–16], and they are found in fresh water resources over a large range of concentrations [17]. Therefore, analytical techniques with high sensitivity are much sought after for the detection and quantification of these NPs. Measurement of the magnitude, frequency, and duration of exposure to these NMs is a critical first step in risk assessment. Unfortunately, it is difficult to build risk assessment scenarios for NMs due to the limited availability of sensitive methods for their detection and quantification.

Analytical methods for the analysis of NMs were recently addressed by Barceló and Farré [18, 19]. They gave a detailed overview of analytical methods and instruments suited for the separation, characterization, and quantification of various NMs and NPs in different matrices. Challenges arise generally from the diversity in their chemical properties and reactivities [20, 21]. Efforts have been focused in our lab to develop polymer growth on NPs as a novel method for their trace analysis in water. The coating material was chosen on the basis of several experimental considerations including simplicity of polymerization in aqueous solution, ability to interact with specific NPs, uniform dispersion of the coated NPs in water, strong ultraviolet (UV) absorptivity, and good electrophoretic mobility for separation by capillary electrophoresis (CE) with UV detection. CE offers various advantages including simplicity in operation, minute sample volumes, minimal consumption of environmentally unfriendly organic solvents, various separation modes, short analysis time, and the high resolution of complex mixtures, which results from having a uniform electroosmotic flow (EOF) [22]. Variance from run to run should not have an effect on the electrophoretic mobility of NMs and NPs as it is measured in relation to a neutral marker, unless there are time-dependent or non-equilibrium interactions of NMs with the capillary wall [23]. Characterization of NPs by CE had been successfully developed in our research lab [24]. Their UV detection limits remained inadequate for general application in environmental science and engineering studies.

In this work, aqueous polymerization of 2-hydroxypropyl methacrylate (HPMA) was investigated on silica (SiO2) for better detection sensitivity in CE-UV analysis. An adhesive coating of polydopamine (PDA) was next put on top of the PHPMA-coated silica nanoparticles (SiO2@PHPMA) to further enhance the detection sensitivity. Attempts were also made to decrease the rate of HPMA polymerization by adding polyvinyl alcohol (PVA) [25], thus increasing the PHPMA coating thickness on SiO2 NPs. Finally, bisphenol A (BPA) was added to increase the UV absorbance of SiO2@PHPMA for extra detection sensitivity.

Material and methods

Materials

2,2′-azobis-2-isobutyronitrile (AIBN) was bought from Pfaltz & Bauer (Waterbury, CT, USA). Bisphenol A (BPA), dopamine hydrochloride (DA.HCl, ≥99.5 %), 2-hydroxypropyl methacrylate (HPMA, 97 %), mesityl oxide (MO, ≥90 %), sodium dodecyl sulfate (SDS, ≥99 %), and LUDOX® colloidal silica (SiO2, 30 % wt. suspension in H2O, with a surface area of 198–250 m2/g), and polyvinyl alcohol (PVA, with an average molecular weight of 10,000 and the degree of hydrolysis about 80 %.) were all purchased from Sigma-Aldrich (Oakville, ON, Canada). Sodium phosphate dibasic was obtained from Fisher Scientific (Fair Lawn, NJ, USA).

Apparatus and analytical method

CE-UV analyses were performed on a modular system built in our laboratory, which includes a Spellman CZE1000R high-voltage power supply (Hauppauge, NY, USA). Fused silica capillary (51 mm i.d., 356 mm o.d.) was obtained from Polymicro Technologies (Phoenix, AZ, USA). The capillary total and effective lengths were 53.5 and 46.1 cm, respectively. The background electrolyte (BGE) was composed of 10 mM Na2HPO4 in deionized distilled water (DDW) to attain pH 7.5 ± 0.2. All CE analyses were run at an applied voltage of 20 kV, with the capillary inlet 2-mm away and below the electrode tip to improve both precision and baseline stability. A Bischoff Lambda 1010 (Leonberg, Germany) UV detector was set up at a wavelength of 190 nm to monitor the SiO2 nanoparticles, polymer(s), and monomer(s). Electrokinetic injection at 17 kV for 12 s was employed to load the sample into the capillary for CE analysis. An independent run of MO as a neutral marker was carried out to determine the ionic charges of SiO2 nanoparticles, polymer, and monomer. A PeakSimple chromatography data system (SRI model 203, Torrance, CA, USA) was used to acquire the detector output signal.

Polymerization of HPMA

Poly (2-hydroxypropyl methacrylate) (PHPMA) was prepared following a procedure developed by Ali et al. [26] with some modification. In a glass vial for free radical polymerization, HPMA (0.007 mol) was first dissolved in DDW (25 mL). SDS (10 wt.% of HPMA) and AIBN (3 wt.% of HPMA) were then added, followed by pure nitrogen gas bubbling for 5 min to remove dissolved oxygen molecules that could destroy the free radicals generated by thermal decomposition of AIBN. Finally, the vial was sealed and placed in a 60 °C thermostatted water bath for 22 h without further mixing or shaking. The polymerization mixture turned cloudy, yielding PHPMA submicron particles with a white color.

PHPMA growth on colloidal SiO2 nanoparticles

HPMA (0.007 mol) was first dissolved in DDW (25 mL) containing colloidal SiO2 nanoparticles (5–20 g/L). Sonication for 5 min was allowed to facilitate their hydrogen bonding interaction. Next, SDS (10 wt.% of HPMA) and AIBN (3 wt.% of HPMA) were added, followed by deoxygenation. Finally, the vial was placed in a 60 °C thermostatted water bath for 22 h to produce PHPMA-coated silica nanoparticles (SiO2@ PHPMA) also with a white color.

Effect of AIBN on PHPMA growth

The amount of AIBN on PHPMA growth on SiO2 nanoparticles was investigated by using 1, 2, and 3 wt.%. The percent conversion to PHPMA was determined by CE analysis for any residual HPMA.

Polydopamine growth on SiO2@ PHPMA

Aqueous solution of DA (200 μL of 25 g/L) was added into SiO2@PHPMA aqueous suspension. The mixture was left alone at ambient temperature (23 ± 2 °C) to allow for PDA growth on the particles, as monitored by CE-UV analysis daily for 1 week.

Polymerization of HPMA with polyvinyl alcohol or bisphenol A

PVA (0.1 g) or BPA (0.2 g/L) was added to the HPMA polymerization mixture to investigate any enhancement of SiO2 detection sensitivity.

Dynamic light scattering

The average diameters of original and polymer-coated SiO2 particles were measured by dynamic light scattering (DLS) using a Brookhaven Instruments NanoDLS particle size analyzer (Holtsville, NY, USA), in ten replicates of 10 s each for higher accuracy.

Transmission electron microscopy

Dry SiO2, SiO2@PHPMA, and SiO2@PHPMA@PDA particles were deposited on a sample holder for transmission electron microscopy (TEM) examination at an accelerating voltage of 120–200 kV using an FEI Tecnai G2 transmission electron microscope (Hillsboro, OR, USA). The diameters of these particles were compared to determine the thickness of different coatings.

Results and discussion

Silica nanoparticles

LUDOX® AM colloidal silica nanoparticles [27] were prepared in the BGE for CE-UV analysis using electrokinetic injection. As shown in Fig. 1, 20 g/L of SiO2 nanoparticles exhibited low UV absorbance at the UV detection wavelength of 190 nm, producing a small peak at the migration time of 7.0 ± 0.1 min.

Fig. 1
figure 1

CE-UV characterization of SiO2 nanoparticles (at 20 g/L) in LUDOX® AM colloid

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The SiO2 nanoparticles appeared after the neutral marker, hence indicating their negative ionic charges (probably as SiO4 2−) in the BGE and yielding a negative electrophoretic mobility value. The standard calibration curve exhibited a linear correlation coefficient (R 2 = 0.979) between their CE-UV peak area and concentration in the working range up to 20 g/L. The limit of detection (LOD at 3σ) and the limit of quantification (LOQ at 10σ) were determined to be 3 and 9 g/L, respectively, which are inadequate for many environmental science and engineering studies.

CE-UV characterization of PHPMA particles

CE-UV analysis was next performed on the HPMA polymerization mixture after 22 h at 60 °C. As a neutral compound illustrated in Fig. 2a, the residual HPMA migrated at nearly the electroosmotic flow velocity to appear at a time close to the 3.7 ± 0.1 min for the neutral marker as shown in Fig. 2b. The late migration time of 5.6 ± 0.1 min for PHPMA indicated its negative ionic charge in the BGE, possibly due to the adsorption of SDS anions on the hydrophobic polymer surface.

Fig. 2
figure 2

CE-UV characterization of a MO and b HPMA (and PHPMA) after 22 h of polymerization

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PHPMA growth on SiO2 particles

Polymerization of HPMA at 60 °C was conducted for 22 h in the presence of SiO2 nanoparticles over a range of concentrations from 5–20 g/L. The resultant SiO2@PHPMA particles were analyzed by CE-UV to determine their electrophoretic mobility and detection sensitivity.

As shown in Fig. 3, a new peak for SiO2@PHPMA particles was detected at a migration time of 6.8 ± 0.1 min. These particles exhibited larger peak height and peak area than the original SiO2 nanoparticles (see Fig. 1). Apparently, the SiO2 nanoparticles were coated by PHPMA to have their Si-O− groups buried under the surface, thus resulting in faster migration than SiO2 nanoparticles (at 7.5 ± 0.1 min) and good separation from PHPMA (at 5.6 ± 0.1 min). Upon varying the concentration of SiO2 nanoparticles in the pre-polymerization mixture, the resultant SiO2@PHPMA peak area changed accordingly as shown in Fig. 4, which provides very convincing proof of the SiO2@PHPMA formation.

Fig. 3
figure 3

CE-UV analysis of PHPMA and SiO2@PHPMA particles after 22 h of polymerization using AIBN at 3 wt.%

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Fig. 4
figure 4

SiO2@PHPMA peak area obtained from different SiO2 concentrations (g/L)

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The LOD and LOQ for SiO2@PHPMA particles were determined to be 0.6 and 1.8 g/L, respectively. Consequently, a 5-fold better CE-UV detection sensitivity was attained for SiO2 nanoparticles after PHPMA coating. Hence, the approach seemed promising toward their sensitive detection in water.

Effect of AIBN on PHPMA growth

Polymerization of HPMA to form a layer of PHPMA on SiO2 nanoparticles was slightly increased from 94 to 96 % when the mount of AIBN (the initiator) was reduced from 3 to 1 wt.%. At 3 wt.%, a larger concentration of reactive radicals would be generated, leading to early termination of the polymerization process and production of a polymer of low molecular weight [28, 29]. On the contrary, at 1 wt.%, a chain carrier will be formed from the reaction of a free radical with a monomer unit and propagation will occur continuously with other monomer units present, resulting in a higher conversion of HPMA to PHPMA. Thereby, larger SiO2@PHPMA particles were produced. A 33-% increase in SiO2@PHPMA peak area was attained and the amount of residual SiO2 nanoparticles was slightly reduced as shown in Fig. 5. Thus, PHPMA growth using AIBN at 1 wt.% resulted in 6–7-fold enhancement of SiO2 detection sensitivity.

Fig. 5
figure 5

CE-UV analysis of PHPMA and SiO2@PHPMA particles after 22 h of polymerization using AIBN at 1 wt.%

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PDA growth on SiO2@PHPMA particles in water

PDA growth on SiO2@PHPMA particles in aqueous suspension over 1 week was investigated for further enhancement of detection sensitivity. The resultant SiO2@PHPMA@PDA particles were analyzed by CE-UV daily. Figure 6 shows the CE-UV electropherogram of DA, HPMA, PHPMA@PDA, SiO2@PHPMA@PDA, and residual SiO2 particles. DA (at 3.2 ± 0.1 min) is positively charged in the BGE as it migrated before the neutral marker. Hence, electrostatic attraction between DA and PHPMA or SiO2@PHPMA particles were expected. Due to the adsorption of SDS anions on the hydrophobic polymer surface, PDA acquired a slight negative charge and migrated behind the neutral marker (as observed in a separate CE-UV analysis). After 7 days of polymerization, the suspension turned black as a result of PDA formation. Both the PHPMA@PDA peak at 7.0 ± 0.1 min and SiO2@PHPMA@PDA peak at 8.1 ± 0.1 min increased in height and area upon PDA growth. Apparently, PHPMA and SiO2@PHPMA particles were coated with a thin layer of PDA to be acquiring extra negative charges on their surfaces, rendering them slower in migration. This thin layer of PDA coating produced an extra 2-fold enhancement of the CE-UV detection sensitivity for SiO2@PHPMA nanoparticles. Thus, with both PHPMA and PDA coatings, a total of 10–14-fold enhancement in detection sensitivity was attained for SiO2 nanoparticles in the original sample.

Fig. 6
figure 6

CE-UV analysis of DA, HPMA, PHPMA@PDA, and SiO2@PHPMA@PDA particles

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Polymerization of HPMA with PVA or BPA

As shown in Table 1, the PHPMA peak areas attained in the presence of SiO2 NPs under different conditions of polymerization were significantly larger than those obtained in the absence of SiO2. The difference in PHPMA peak areas between the presence and absence of SiO2 NPs was also significantly larger than the peak area of SiO2@PHPMA. One speculation for this significant difference is the catalytic effect of SiO2 NPs that could facilitate the HPMA polymerization and thus produce more PHPMA particles. Another speculation is the formation of a thick coating of PHPMA to cover up all the negative charges present on the SiO2 surface, thus producing SiO2@PHPMA particles with effectively the same migration time as PHPMA particles. Nonetheless, the SiO2@PHPMA peak area and height were significantly larger than those of the original SiO2 NPs. These SiO2@PHPMA particles might have only a monolayer of PHPMA that could not cover up all the negative charges on SiO2, and thus, they migrated after the PHPMA particles. Chu et al. had previously used PVA to decrease the rate of HPMA polymerization, thereby producing larger particles [25]. In our study, PVA increased the PHPMA peak area, which indicates the formation of larger PHPMA particles as confirmed by DLS analysis below. Addition of BPA also resulted in the formation of larger PHPMA particles. Consequently, PVA or BPA addition to the HPMA polymerization mixture containing SiO2 NPs resulted in 9–10-fold increase of SiO2 detection sensitivity probably due to additional absorption of the UV detector light.

Table 1 CE-UV peak areas of HPMA, PHPMA, and SiO2@PHPMA under different conditions of HPMA polymerization. All peak areas are expressed in arbitrary units of mV.s
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Dynamic light scattering

Aqueous suspensions of original and polymer-coated SiO2 particles were analyzed by DLS to determine their hydrodynamic diameters to gain some insight of their growth. The hydrodynamic diameter represents the actual particle diameter plus the hydration layer surrounding the particle as it moves under the influence of Brownian motion. As shown in Fig. 7a, b, mean diameters of 53 ± 3 and 77 ± 5 nm were obtained for SiO2 nanoparticles and PHPMA particles, respectively. Larger particle diameters were exhibited by SiO2@PHPMA, SiO2@PHPMA@PDA, and SiO2@PHPMA formed in the presence of BPA and PVA as summarized in Table 2. These results provide strong evidence that SiO2 nanoparticles in water can be made larger in size by polymer growth under simple experimental conditions to offer more sensitive detection by CE-UV, with the option of electrophoretic separation from organic compounds possibly found in water. In principle, our encapsulation technique (based on the polymerization of HPMA) may be affected by other components in the aqueous solution. However, it can be applied to real sample analysis. Samples would be prepared prior to polymerization to coat NPs. A filter of appropriate porosity can be used to remove soil particulates found in the water sample. The filtrate pH can be adjusted to the desired pH before starting the polymerization. The effect of salt will be removed in our future work by passing through a column of ion-exchange resin.

Fig. 7
figure 7

Dynamic light scattering measurements of a lognormal distribution of SiO2 nanoparticles in LUDOX® AM colloid and b lognormal distribution of PHPMA in DDW after 22 h of polymerization at 60 °C

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Table 2 DLS measurement of the hydrodynamic diameters of original and polymer-coated SiO2 particles
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Transmission electron microscopy

Figure 8 shows the TEM images for SiO2@PHPMA and SiO2@PHPMA@PDA particles. A mean diameter of 14.6 ± 2.9 nm was found for SiO2 nanoparticles. As shown in Fig. 8a, b, mean diameters of 18.6 ± 2.2 and 20.1 ± 1.6 nm were obtained for SiO2@PHPMA and SiO2@PHPMA@PDA, respectively. The thickness of PHPMA coating on the surface of SiO2 nanoparticles was approximately 2 ± 0.4 nm, whereas PDA coating on top of PHPMA was about 0.8 ± 0.3 nm. The diameters of these dry particles, as determined by TEM, seem to be much smaller than the corresponding hydrodynamic diameters obtained by DLS measurement for particles in aqueous suspensions. Particles in liquid media develop a hydration layer around their surfaces, which impacts on their movement under the influence of Brownian motion. As this hydration layer is not present in TEM analysis, smaller diameters were obtained.

Fig. 8
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Transmission electron micrographs of a SiO2@PHPMA and b SiO2@PHPMA@PDA

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Conclusions

This work has demonstrated a new analytical approach to enhance the sensitivity for CE-UV detection of colloidal SiO2 nanoparticles in water by coating with PHPMA alone or with the assistance of PDA, BPA, and PVA. The problem of inadequate sensitivity was broken down into smaller attempts toward technically simple and operationally cost-effective solutions. Experimental results have demonstrated the feasibility of an approach that can potentially be applied for the determination of other NMs and NPs in water analysis. Controlled growth of thicker polymer coatings is underway in our lab to attain complete coating, which would offer higher sensitivity needed for the detection of lower concentrations of NPs. Our ultimate objective is the implementation of this new approach in toxicology research regarding nanomaterials in water, to gain a better understanding of their long-term impact on environmental sustainability and public health.