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Application of surface-modified silica nanoparticles with dual silane coupling agents in bitumen for performance enhancement
Construction and Building Materials 244 (2020) 118324
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Application of surface-modified silica nanoparticles with dual silane coupling agents in bitumen for performance enhancement Sidharth Reddy Karnati a, Daniel Oldham b, Elham H. Fini b,⇑, Lifeng Zhang a,⇑ a b
Department of Nanoengineering, Joint School of Nanoscience and Nanoengineering, North Carolina A&T State University, Greensboro, NC 27401, USA School of Sustainable Engineering and Built Environment, Arizona State University, Tempe, AZ 85287, USA
h i g h l i g h t s The best result for SNP surface modification was from dual silane APTES-GPTMS. APTES-GPTMS-SNPs outperformed the reported APTES-SNPs for bitumen modification. The bitumen with APTES-GPTMS-SNPs had more resistance to aging, rutting & cracking. The enhanced bitumen was ascribed to better dispersion of APTES-GPTMS-SNPs. The result provides insight on bitumen enhancement particularly for delay of aging.
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Article history: Received 27 July 2019 Received in revised form 14 December 2019 Accepted 30 January 2020
Keywords: Silica nanoparticles Surface modification Silane coupling agent Dispersion Bitumen Anti-aging
a b s t r a c t Bitumen is a black and highly viscous liquid that holds stone aggregate together in road pavement. Oxidation aging of bitumen can accelerate overall pavement deterioration and shorten pavement service life. Use of silica nanoparticles (SNPs) have been promoted as a sustainable construction practice to delay bitumen oxidation. Nonetheless, adequate dispersion of SNPs in bitumen has been a challenge since their first employment. Surface functionalization of SNPs with (3-aminopropyl) triethoxysilane (APTES) has demonstrated its efficiency to improve dispersion of SNPs in bitumen at relatively high loading of SNPs with low mechanical energy input. As the amount of APTES increases in the process of surface modification of SNPs, however, average size of the resultant SNPs and their agglomeration in bitumen also increase due to APTES self-condensation reaction on SNP surface. This research investigated surface modification of SNPs with other silane coupling agents including 3-(trihydroxysilyl) propyl methylphosphonate (THPMP) and (3-glycidyloxypropyl) trimethoxysilane (GPTMS) as well as dual silane combinations: APTES with THPMP and APTES with GPTMS. The intention was to minimize agglomeration of SNPs in bitumen and to further improve overall performance of SNP-containing bitumen. The comparative experimental results indicated that the surface-modified SNPs with dual silanes (APTES-GPTMS) outperformed the sole APTES-modified SNPs regarding dispersion in bitumen, resulting in bitumen with more enhanced anti-aging and low temperature properties. The results of this study inform and promote the application of SNPs in road pavement with enhanced performance and sustainability. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, organic/inorganic nanomaterials such as carbon nanoparticles, silica nanoparticles, nanoclay, and carbon nanotubes have been used as modifiers of bitumen (sometimes termed as asphalt binder) to improve its performance [1–4]. Among all inorganic nanofillers, silica (SiO2) nanoparticles (SNPs) demonstrated great improvement in performance of bitumen including aging, ⇑ Corresponding authors. E-mail addresses: [email protected] (E.H. Fini), [email protected] (L. Zhang). https://doi.org/10.1016/j.conbuildmat.2020.118324 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
fatigue cracking, and rutting properties, but negatively impacted low temperature properties [5]. It is shown from previous studies that performance of the SNP-modified bitumen degraded when loading of SNPs went beyond 3 wt% even being mixed at high shearing speed (4000–5000 rpm), long mixing time (1–2 h) and high mixing temperature (> 135 °C) [2,5–9]. This degradation can be attributed to agglomeration of SNPs and their poor dispersion in bitumen at high loading. It has been well documented that surface modification of SNPs plays a vital role in improving their dispersion in polymer matrix as well as strengthening SNP-polymer matrix interface and has
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resulted in polymer nanocomposites with higher performance [10– 12]. (3-Aminopropyl) triethoxysilane (APTES) is a common silane coupling agent that is used for surface modification of SNPs and has been applied in different applications like polymer nanocomposites, drug delivery, and biosensing [13–15]. In our most recent research, APTES surface-modified SNPs (APTES-SNPs) was introduced to bitumen at 4% dosage by weight of bitumen [16]. Compared to the bitumen with pristine SNPs, the bitumen with APTES-SNPs allowed improvement in dispersion of SNPs in bitumen matrix and led to significant enhancement of bitumen’s resistance to aging, rutting as well as fatigue and low temperature cracking. It should be noted that as the amount of APTES increases in the process of surface modification of SNPs, average size of the resultant surface-modified SNPs and their agglomeration in bitumen increase due to APTES self-condensation reaction on SNP surface [17]. Large amount of APTES could also reduce stability of individual SNPs as indicated by average zeta potential of the surface modified SNPs [16]. This research aims to reduce agglomeration of surface-modified SNPs in bitumen by preventing self-condensation reaction of APTES through application of other silane coupling agents as well as through dual silane application. 3-(Trihydroxysilyl) propyl methylphosphonate (THPMP) and (3-Glycidyloxypropyl) trimethoxysilane (GPTMS) are two other common silane coupling agents. It is hypothesized that charge-bearing THPMP may help stabilize surface-modified SNPs through electrical repulsive force [18]. Considering that THPMP can make the surface of SNPs more hydrophilic [19,20] and GPTMS can make the surface of SNPs more hydrophobic [21–23], a comparative study on SNP surface modification and resultant bitumen performance may lead to a general guideline for tailoring surface modification of SNPs to be used in bitumen for road pavements. Specifically the effects of surface-modified SNPs on performance of bitumen were investigated and compared in this research. The surface modification of SNPs was confirmed by Fourier-transform infrared spectroscopy (FTIR). The morphology, size and stability of surface-modified SNPs were characterized using scanning electron microscope (SEM) and dynamic light scattering (DLS). Performance of bitumen with these surface-modified SNPs at 4 wt% loading was evaluated by aging, rutting, fatigue cracking, and low-temperature cracking. The relationship between surface modification of SNPs with different silanes/silane combinations and performance of the resultant modified bitumen was revealed. 2. Materials and methods 2.1. Materials The bitumen (PG 64-22, Table 1) in this research was acquired from Associated Asphalt in Greensboro, NC. (3-aminopropyl) triethoxysilane (APTES, catalog number A3648), 3-(trihydroxysilyl) propyl methylphosphonate (THPMP, catalog number 435716), (3glycidyloxypropyl) trimethoxysilane (GPTMS, catalog number 440167) and SiO2 nanoparticles (SNPs, catalog number 718483) with average size of 12 nm and 175–225 m2/g BET surface area
Table 1 General properties of bitumen PG 64-22. Specific Gravity @15.6 °C Flash Point, Cleveland Open Cup, °C Change in Mass RTFO Absolute Viscosity at 60 °C, Pas Stiffness (MPa) at 12 °C @ 60 s
1.039 335 0.0129 202 112.5
were purchased from Sigma Aldrich. Ethanol was purchased from Fisher Scientific. 2.2. Sample preparation 2.2.1. Surface modification of SNPs SNPs were first added in 95/5 (weight) ethanol/water mixture with stirring and then sonicated for 10 min to achieve good dispersion. To surface-modify pristine SNPs, individual silane coupling agent including APTES, THPMP, and GPTMS was added to the system, respectively, at a weight ratio of silane/SNP = 1.6 and sonicated for another 5 min. The mixture was further stirred for 2 h and maintained at 70 °C to obtain respective surface-modified SNPs with APTES (APTES-SNPs), with THPMP (THPMP-SNPs), and with GPTMS (GPTMS-SNPs). To further surface-modify APTESSNPs with GPTMS or THPMP (i.e. dual-silane modification), same surface-modification procedure as described above was followed except using APTES-SNPs as starting material. GPTMS or THPMP was added to the reaction system with APTES-SNPs and the whole system was stirred for another 2 h and maintained at 70 °C to obtain surface-modified SNPs with APTES and THPMP (APTESTHPMP-SNPs) and with APTES and GPTMS (APTES-GPTMS-SNPs), respectively. All the surface-modified SNPs including APTES-SNPs, THPMP-SNPs, GPTMS-SNPs, APTES-THPMP-SNPs, and APTESGPTMS-SNPs were individually separated by centrifugation, washed with fresh DI water thoroughly, and then freeze-dried for 24 h before application in bitumen. 2.2.2. Mixing SNPs with bitumen The bitumen was initially heated to 135 °C. Then 4 wt% of certain surface-modified SNPs were added and mixed with the bitumen using a mechanical stirring at 900 rpm for 30 min at 135 °C. 2.2.3. Aging of asphalt binder Aging of modified bitumen with respective silane-modified SNPs was performed through two stages: short-term aging and long-term aging. Rolling thin film oven (RTFO) procedure is used to perform short-term aging of unaged bitumen according to ASTM D 2872 at 163 °C for 85 min in the presence of constant air flow followed by long-term aging in a pressure aging vessel (PAV, Prentex Model 9300) based on ASTM D6521 at 100 °C under 300 psi for 20 h. 2.3. Characterization Surface modification of SNPs with silane coupling agents was characterized by a Varian 670 FTIR spectrometer. Hydrodynamic diameter and zeta potential of surface-modified SNPs were characterized by using dynamic light scattering (DLS, Malvern Zetasizer Nano ZS). For each SNP sample, 0.1 mg SNPs were sonicated for 5 min in 1 L of DI water and 1 ml of the water with SNPs was taken out afterwards to perform DLS measurement. A Zeiss Auriga Crossbeam FIB Field Emission Scanning Electron Microscope (FESEM) was used to characterize morphology of SNPs. A gold–palladium layer of 4 nm was sputter-coated on all SNP samples to improve image quality. A Zeiss Evo Environmental SEM with cool-stage was employed to characterize morphology and distribution of SNPs in bitumen. Characterization of bitumen samples including viscosity, aging and low temperature properties followed the same procedure as reported elsewhere [16]. Specifically, viscosity of bitumen samples was measured using a Brookfield DV-III ultra-rotational viscometer (RV) in accordance with ASTM D4402 at 135 °C with a speed of 20 rpm. Viscosity aging index (VAI) was calculated using viscosity of each bitumen sample before and after long-term aging. FTIR was used to monitor carbonyl and sulfoxide functional groups of
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unaged and aged bitumen samples to investigate oxidative aging behavior of bitumen with surface-modified SNPs. The IR absorbance peaks around 1700 cm 1 and 1030 cm 1 are attributed to be carbonyl and sulfoxide functional groups, respectively. Corresponding carbonyl or sulfoxide peak area in IR spectrum of a specific bitumen sample with surface-modified SNPs was divided by the entire area between 2000 and 600 cm 1 in the IR spectrum to calculate chemical aging indices (CAI) of both unaged and aged samples. A Kinexus dynamic shear rheometer (DSR-III, HM-87) was used to characterize both viscous and elastic behavior of bitumen samples by measuring complex shear modulus (G*) and phase angle (d) according to ASTM D7175-15. The DSR data for rutting resistance (G*/Sin d) evaluation were collected at 64 °C while the DSR data for fatigue cracking (G* Sin d) evaluation were collected at 28 °C. The rheological aging index (RAI) of each bitumen sample was determined using the results at 64 °C. The selected DSR frequency for these samples was 10 rad/s. Low temperature properties of bitumen samples with surface-modified SNPs including ductility and fracture energy were evaluated by a Direct Tension Tester (Interlaken) following ASTM 6723–12. The test was conducted at 12 °C with a displacement rate of 1.00 mm/min. Fracture energy was determined by the area under the load-displacement curve and ductility was determined based on elongation at break.
3. Results and discussion 3.1. Surface modification of SNPs with single and dual silanes FTIR was used to characterize surface modification of SNPs with THPMP, GPTMS as well as dual silanes (APTES + THPMP and APTES + GPTMS) (Fig. 1). In comparison with pristine SNPs, the IR spectrum of THPMP-SNPs exhibited peaks around ~2885 cm 1/ ~2940 cm 1 and ~1420 cm 1/~1350 cm 1, which are typical absorption peaks attributed to stretching and bending vibrations of C-H in –CH2– and –CH3 [24] and an evidence for successful attachment of THPMP on SNPs (Fig. 1a). The peak centered at ~1310 cm 1 is attributed to P = O stretching [25,26] and further confirmed the presence of THPMP on surface of SNPs [20] (Fig. 1a). Similarly, the IR spectrum of GPTMS-SNPs showed peaks at ~2935 cm 1/~2920 cm 1/~2900 cm 1/~2875 cm 1/~2840 cm 1 and ~1425 cm 1/~1360 cm 1, which are attributed to stretching and bending vibrations of C-H in –CH2– and –CH< in GPTMS molecules, indicating successful attachment of GPTMS on SNPs. The enhanced peak centered at ~810 cm 1 indicated the presence of C-O in epoxide functional groups. All of these observations confirmed the graft of GPTMS on surface of SNPs. FTIR spectra of pristine SNPs, APTES-SNPs, as well as dual-silane treated SNPs including APTES-THPMP-SNPs and APTES-GPTMSSNPs are compared in Fig. 1b. All the silane treated SNPs showed typical –CH3 and –CH2– peaks as described before. The FTIR spectrum of APTES-SNPs showed a peak centered at ~1590 cm 1 due to N–H bending in primary amine –NH2, indicating successful grafting of APTES on SNP surface [16]. Based on the FTIR spectrum of APTES-SNPs, it is observed that the characteristic peak of primary amine –NH2 disappeared in IR spectrum of APTES-THPMP-SNPs sample while the characteristic peak of P = O at ~1310 cm 1 was present, which indicated some reaction between APTES and THPMP on SNP surface. Since the graft of APTES on SNP surface was previously confirmed and THPMP was added after APTES functionalization, the presence of THPMP characteristic peaks suggested co-existence of APTES and THPMP on surface of SNPs. As for APTES-GPTMS-SNPs sample, the FTIR spectrum also showed disappearance of primary amine characteristic peak while an enhanced peak at ~1410 cm 1 could be assigned to O–H bending
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in tertiary alcohol. This is a proof for the chemical reaction between APTES and GPTMS where epoxide rings in GPTMS molecules were opened by the amine functional groups in APTES, indicating APTES and GPTMS structures are connected and co-exist on SNP surface. The enhanced peaks in the region of ~3100 cm 1 and ~3500 cm 1 from FTIR spectra of APTES-THPMP-SNPs and APTESGPTMS-SNPs are attributed to –OH from THPMP and formation of tertiary alcohol due to epoxide ring opening from GPTMS, respectively, which is another evidence for the reactions between APTES and THPMP and between APTES and GPTMS, respectively. The possible chemical reaction mechanisms between SNPs and silane coupling agents are demonstrated in Fig. 2. To evaluate stability and size of various silane-treated SNPs, average size and zeta potential of all surface modified SNPs were characterized by using DLS and compared with those of APTESSNPs (Fig. 3). The average size and zeta potential of APTES-SNPs were 269 nm & 11.5 mV. Compared to APTES, employment of THPMP increased average size of the resultant surface-modified SNPs by 36% and reduced average zeta potential of the SNPs by 14% while grafting GPTMS on SNPs increased average size of the resultant surface-modified SNP by 11% and increased average zeta potential of the SNPs by 152%. In the case of dual silanes, compared to APTES, APTES-THPMP increased average size of the reultant surface-modified SNPs by 9% and increased average zeta potential of the SNPs by 48% while APTES-GPTMS reduced average size of the resultant surface-modified SNPs by 19% and increased average zeta potential of the SNPs by 123%. From the point of view of stability (larger absolute value of zeta potential) and less agglomeration (smaller avearge size), the dual silanes with APTES and GPTMS demonstrated the best overall result followed by GPTMS alone. In other words, GPTMS can be used to replace APTES to modify pristine SNPs or further modify APTES-SNPs for better particle stability and smaller particle size. Compared to the case of APTES where positive ions (–NH+3) and negative ions (–Si-O-) together determines overall zeta potential of the SNPs, the carbon atoms in epoxide groups of GPTMS are electrophiles and can attract negative charges. In combination with the negative ions of –Si-O- on surface of SNPs, GPTMS-SNPs exhibited the overall largest negative charges, which has the greatest potential to facilitate dispersion and stabilization of SNPs in bitumen. 3.2. Comparative performance evaluation of bitumen samples with silane surface-modified SNPs 3.2.1. Aging evaluation Surface-modified SNPs were individually added to respective bitumen samples at 4 wt% loading and their effects on aging of the bitumen were examined using viscosity aging index (VAI), chemical aging index (CAI) including carbonyl index and sulfoxide index, and rheological aging index (RAI) (Fig. 4). Viscosity of the bitumen PG 64–22 containing 4 wt% APTESSNPs was found to be 36% and 63% less than that of the bitumen containing the same amount of pristine SNPs before and after aging [16]. Compared to APTES-SNPs, employment of THPMP-SNPs, GPTMS-SNPs, and APTES-THPMP-SNPs reduced the bitumen’s viscosity before aging by 7%, 2%, and 12%, respectively, while APTES-GPTMS-SNPs increased the bitumen’s viscosity before aging by nearly 22%. Compared to the bitumen with APTES-SNPs, viscosities of bitumen samples after aging increased by 2% and 7% with THPMP-SNPs and APTES-THPMP-SNPs, respectively, but reduced by 11% and 24%, respectively, with GPTMS-SNPs and APTESGPTMS-SNPs. Compared to APTES-SNPs, THPMP-SNPs and APTES-THPMPSNPs increased VAI of the bitumen by 12% and 29%, respectively, while GPTMS-SNPs and APTES-GPTMS-SNPs reduced it by 13% and 51%, respectively. As evidenced by better particle stability
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Fig. 1. FTIR spectra of pristine SNPs and surface-modified SNPs with THPMP, GPTMS, and APTES based dual silanes.
and smaller particle size (Fig. 3), GPTMS-containing SNPs could have better distribution in bitumen matrix and thus have more interaction between molecules in bitumen and surface-modified SNPs, reducing internal friction between bitumen molecules (lowering the viscosity) and chance of reaction between bitumen and oxygen (more aging resistance). On the contrary, THPMPcontaining SNPs worsened the oxidative aging of the bitumen due to their relatively poor distribution in bitumen matrix caused by larger particle size and lower zeta potential as well as reduced compatibility between THPMP-SNPs and overall hydrophobic bitumen based on more hydrophilicity of THPMP than APTES. Compared to the bitumen with APTES-SNPs, the bitumen containing THPMP-SNPs and APTES-THPMP-SNPs showed 24% and
26% more carbonyl CAI and 452% and 375% more sulfoxide CAI, respectively, while the bitumen with GPTMS-SNPs and APTESGPTMS-SNPs reached 31% and 23% less carbonyl CAI but 289% and 105% more sulfoxide CAI, respectively. The smallest sulfoxide CAI thus occurred with APTES. Interactions such as hydrogen bonding and/or dipole-dipole interaction between APTES molecules on APTES-SNPs and molecules in bitumen could reduce chance of oxidative reaction between the bitumen molecules and environmental oxygen, thus lowering amount of carbonyl and sulfoxide functional groups during oxidation and reducing CAI [16]. GPTMS-containing SNPs showed reduction in carbonyl CAI compared to APTES-SNPs possibly due to more oxygen atoms in GPTMS molecular structure for hydrogen bonding and/or dipole-dipole interaction with bitumen molecules. It is noteworthy that sulfur
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Fig. 2. Schematic illustration of possible reactions between SNPs and silane coupling agents.
has stronger electronegativity than carbon while sulfoxide is more polar than carbonyl group. According to chemical structures of the silanes in this research (Fig. 2), there were more chances for –NH2 in APTES than epoxide functional group in GPTMS to form hydrogen bonding and/or dipole-dipole interaction with sulfurcontaining molecules in bitumen. The THPMP-containing SNPs may be able to form hydrogen bonding and/or dipole-dipole interaction with polar groups in bitumen as well, but their relatively bigger size and less homogeneous distribution in bitumen could counteract the abovementioned interaction and thus lead to increased carbonyl CAI and much larger sulfoxide CAI. RAI characterizes oxidative aging of bitumen based on nonrecoverable deformation and hardening. Compared to the bitumen sample with APTES-SNPs, the bitumen samples with THPMP-SNPs and APTES-THPMP-SNPs exhibited 25% and 32% increase in RAI, respectively, while the bitumen samples with GPTMS-SNPs and APTES-GPTMS-SNPs presented 16% reduction and 2% increase in RAI, respectively. The RAI results further demonstrated that THPMP is not a good surface-modifying agent for SNPs from anti-
aging perspective while GPTMS performed better compared to APTES. 3.2.2. Rutting resistance, fatigue cracking and low temperature cracking properties Combination of complex shear modulus (G*) and phase angle (d) from DSR measurement was used to characterize rutting resistance (G*/Sin d) and fatigue cracking (G* Sin d) of bitumen samples containing surface-modified SNPs with single and dual silanes. Higher value of G*/Sin d indicates higher resistance to permanent deformation (rutting) in asphalt pavement. Lower value of G* Sin d suggests higher resistance to fatigue cracking in asphalt pavement. Experimental results (Fig. 5) revealed that only the bitumen sample containing APTES-GPTMS-SNPs had higher rutting resistance than that of the one containing APTES-SNPs, which reflected a 7% improvement. Rutting resistance of the bitumen samples containing THPMP-SNPs, GPTMS-SNPs, APTES-THPMPSNPs decreased by 10%, 12%, and 17%, respectively, compared to that of the bitumen containing APTES-SNPs.
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Fatigue cracking of bitumen samples containing GPTMSSNPs and APTES-GPTMS-SNPs was reduced by 27% and 33%, respectively, compared to that of the bitumen sample containing APTES-SNPs. However, fatigue cracking of the bitumen samples containing THPMP-SNPs and APTES-THPMP-SNPs increased by 16% and 12%, respectively. From the point of view of rutting resistance and reduction of fatigue cracking, APTESGPTMS-SNPs performed the best among all scenarios studied here. Low temperature performance of unaged and aged bitumen samples with APTES-SNPs, GPTMS-SNPs and APTES-GPTMSSNPs were further characterized by measuring their fracture energy and ductility (Fig. 6). As for unaged samples, the bitumen samples with GPTMS-SNPs and APTES-GPTMS-SNPs exhibited a reduction of 10% and 5% in fracture energy, respectively, compared to the bitumen sample containing APTES-SNPs. However, fracture energy of bitumen samples with APTES-GPTMS-SNPs and GPTMS-SNPs after aging was found to be 5% higher and 60% lower, respectively, than that of the bitumen sample containing APTES-SNPs. Compared to the sample containing APTES-SNPs, the bitumen sample with APTES-GPTMS-SNPs showed similar ductility while the bitumen sample with
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Fig. 4. Aging characterization of bitumen samples containing 4 wt% surface-modified SNPs with single and dual silanes: (a) viscosities before and after long-term aging; (b) long-term viscosity aging indices; (c) carbonyl and sulfoxide chemical aging indices; (d) rheological aging indices.
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Fig. 5. Rutting resistance and fatigue cracking of bitumen samples containing 4 wt% surface-modified SNPs with single and dual silanes.
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Fig. 6. Fracture toughness and ductility of unaged and aged bitumen samples containing 4 wt% surface-modified SNPs with single and dual silane coupling agents.
GPTMS-SNPs presented lower ductility for both unaged and aged samples: 15% reduction for the unaged sample and 41% reduction for the aged sample. Overall, the aged bitumen containing APTES-GPTMS-SNPs slightly outperformed the aged bitumen containing APTES-SNPs in low temperature properties. Based on all the experimental data herein, performance of silane-modified SNPs in bitumen can be ascribed to two factors: (1) SNP distribution in bitumen matrix; (2) interfacial strength between SNP and bitumen matrix. Stronger interfacial bonding between SNPs and bitumen matrix requires more energy to undergo de-bonding and thus improve mechanical properties of bitumen as indicated in polymer nanocomposite [13]. According to SNP morphology from SEM, GPTMS-SNPs and APTES-GPTMSSNPs showed much smaller size and more homogeneous distribution in bitumen matrix than APTES-SNPs (Fig. 7). APTESGPTMS-SNPs performed overall the best among all silane surfacemodified SNPs in this research and particularly outperformed APTES-SNPs, which was investigated in previous study and demon-
strated their effectiveness in property enhancement of bitumen [16]. The observed bitumen property improvement from APTESGPTMS-SNPs can be attributed to their better dispersion in bitumen matrix than APTES-SNPs due to the break of APTES selfcondensation reaction through GPTMS reaction with APTES. The dispersion of APTES-GPTMS-SNPs in bitumen matrix could be further facilitated by enhanced compatibility of the SNPs with bitumen due to more hydrophobic nature of the SNP surface from reaction between APTES and GPTMS than that with only APTES. Compared to APTES-SNPs, the improvement in compatibility of APTES-GPTMS-SNPs with bitumen not only ensured enough chance to form hydrogen bonding and/or dipole–dipole interaction with polar groups in bitumen to reduce chance of reaction between molecules in bitumen and environmental oxygen, but also simultaneously provided better interfacial bonding between SNPs and bitumen matrix. Consequently, the dual silane (APTES-GPTMS) surface modified SNPs demonstrated more enhanced anti-aging resistance and low temperature properties.
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Fig. 7. SEM images showing distribution of APTES-SNPs (a), GPTMS-SNPs (b), and APTES-GPTMS-SNPs (c) in bitumen.
4. Conclusion
Acknowledgement
Surface modification of silica nanoparticles (SNPs) with silane coupling agents including 3-(trihydroxysilyl) propyl methylphosphonate (THPMP), (3-glycidyloxypropyl) trimethoxysilane (GPTMS), and dual silane combinations of (3-aminopropyl) triethoxysilane (APTES) with THPMP and APTES with GPTMS was successfully carried out. Surface-modified SNPs were then introduced to bitumen (PG 64-22) at 4 wt% loading under relatively low mixing temperature (135 °C), low shear rate (900 rpm), and short mixing time (30 min). Among all the investigated silanes and silane combinations, the combination of APTES with GPTMS demonstrated the best result from the points of view of stability, agglomeration, distribution in bitumen, and property enhancement of bitumen. APTES-GPTMSSNPs outperformed the previously reported APTES-SNPs for application in bitumen and rendered bitumen more resistance to aging, rutting, fatigue cracking and low temperature cracking. Compared to APTES-SNPs, specifically, APTES-GPTMS-SNPs reduced particle average size by 19% and increased particle average zeta potential by 123%. Further comparison revealed that viscosity aging index (VAI) and carbonyl chemical aging index (CAI) of the bitumen was lowered by 51% and by 23%, respectively, when APTES-GPTMSSNPs was employed instead of APTES-SNPs. In addition, the resistance to rutting, low temperature cracking, and fatigue cracking was 7%, 5%, and 33%, respectively, higher for the bitumen sample with APTES-GPTMS-SNPs than the one with APTES-SNPs. Compared to APTES-SNPs, the observed bitumen property improvements from APTES-GPTMS-SNPs implementation was attributed to better dispersion of this dual-silane treated SNPs, which was mainly resulted from the reduction of APTES selfcondensation reaction through the reaction between GPTMS and APTES as well as increased compatibility between APTES-GPTMSSNPs and bitumen due to more hydrophobic nature of APTESGPTMS-SNPs with respect to APTES-SNPs. It is suggested that surface-modification of SNPs can be judiciously adjusted to maximize the enhancement of bitumen for road pavement.
The authors thank the financial support from the Joint School of Nanoscience and Nanoengineering of North Carolina A&T State University, a member of Southeastern Nanotechnology Infrastructure Corridor (SENIC) and National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (ECCS-1542174).
CRediT authorship contribution statement Sidharth Reddy Karnati: Investigation, Formal analysis, Writing - original draft. Daniel Oldham: Investigation, Data curation, Validation. Elham H. Fini: Conceptualization, Methodology, Writing - review & editing. Lifeng Zhang: Conceptualization, Resources, Writing - review & editing, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
References [1] F. Xiao, A.N. Amirkhanian, S.N. Amirkhanian, Influence of carbon nanoparticles on the rheological characteristics of short-term aged asphalt binders, J. Mater. Civ. Eng. 23 (2011) 423–431, https://doi.org/10.1061/(ASCE)MT.19435533.0000184. [2] E.H. Fini, P. Hajikarimi, R. Mohammad, F.M. Nejad, Physiochemical, rheological, and oxidative aging characteristics of asphalt binder in the presence of mesoporous silica nanoparticles, J. Mater. Civ. Eng. 28 (2016) 04015133, https://doi.org/10.1061/(ASCE)MT.1943-5533.0001423. [3] M. Abdelrahman, D.R. Katti, A. Ghavibazoo, H.B. Upadhyay, K.S. Katti, Engineering physical properties of asphalt binders through nanoclay–asphalt interactions, J. Mater. Civ. Eng. 26 (2014) 04014099, https://doi.org/10.1061/ (ASCE)MT.1943-5533.0001017. [4] M. Faramarzi, M. Arabani, A.K. Haghi, V. Mottaghitalab, Carbon nanotubesmodified asphalt binder: preparation and characterization, Int. J. Pavement Res. Technol. 8 (2015) 29–37, https://doi.org/10.6135/ijprt.org.tw/2015.8 (1).29. [5] H. Yao, Z. You, L. Li, C.H. Lee, D. Wingard, Y.K. Yap, X. Shi, S.W. Goh, Rheological properties and chemical bonding of asphalt modified with nanosilica, J. Mater. Civ. Eng. 25 (2013) 1619–1630, https://doi.org/10.1061/(ASCE)MT.19435533.0000690. [6] Z. You, J. Mills-Beale, J.M. Foley, S. Roy, G.M. Odegard, Q. Dai, S.W. Goh, Nanoclay-modified asphalt materials: preparation and characterization, Constr. Build. Mater. 25 (2011) 1072–1078, https://doi.org/10.1016/ j.conbuildmat.2010.06.070. [7] N.I.M. Yusoff, A.A.S. Breem, H.N.M. Alattug, A. Hamim, J. Ahmad, The effects of moisture susceptibility and ageing conditions on nano-silica/polymermodified asphalt mixtures, Constr. Build. Mater. 72 (2014) 139–147, https:// doi.org/10.1016/j.conbuildmat.2014.09.014. [8] G.M. Amin, A. Esmail, Application of nano silica to improve self-healing of asphalt mixes, J. Cent. South Univ. 24 (2017) 1019–1026, https://doi.org/ 10.1007/s11771-017-3504-y. [9] X. Guo, M. Sun, W. Dai, S. Chen, Performance characteristics of silane silica modified asphalt, Adv. Mater. Sci. Eng. 2016 (2016) 6731232, https://doi.org/ 10.1155/2016/6731232. [10] M. Zamanian, M. Mortezaei, B. Salehnia, J.E. Jam, Fracture toughness of epoxy polymer modified with nanosilica particles: Particle size effect, Eng. Fract. Mech. 97 (2013) 193–206, https://doi.org/10.1016/j.engfracmech.2012.10.027. [11] M.Z. Rong, M.Q. Zhang, W.H. Ruan, Surface modification of nanoscale fillers for improving properties of polymer nanocomposites: a review, Mater. Sci. Technol. 22 (2006) 787–796, https://doi.org/10.1179/174328406X101247. [12] A. Hirsch, Functionalization of single-walled carbon nanotubes, Angew. Chem. Int. Ed. 41 (2002) 1853–1859, https://doi.org/10.1002/1521-3773(20020603) 41:11<1853::AID-ANIE1853>3.0.CO;2-N. [13] G. Wang, D. Yu, R.V. Mohan, S. Gbewonyo, L. Zhang, A comparative study of nanoscale glass filler reinforced epoxy composites: electrospun nanofiber vs nanoparticle, Compos. Sci. Technol. 129 (2016) 19–29, https://doi.org/ 10.1016/j.compscitech.2016.04.006. [14] F. Tang, L. Li, D. Chen, Mesoporous silica nanoparticles: synthesis, biocompatibility and drug delivery, Adv. Mater. 24 (2012) 1504–1534, https://doi.org/10.1002/adma.201104763. [15] K. De Vos, I. Bartolozzi, E. Schacht, P. Bienstman, R. Baets, Silicon-on-Insulator microring resonator for sensitive and label-free biosensing, Opt. Express 15 (2007) 7610–7615, https://doi.org/10.1364/OE.15.007610.
S.R. Karnati et al. / Construction and Building Materials 244 (2020) 118324 [16] S.R. Karnati, D. Oldham, E.H. Fini, L. Zhang, Surface functionalization of silica nanoparticles to enhance aging resistance of asphalt binder, Constr. Build. Mater. 211 (2019) 1065–1072, https://doi.org/10.1016/ j.conbuildmat.2019.03.257. [17] I.A. Rahman, M. Jafarzadeh, C.S. Sipaut, Synthesis of organo-functionalized nanosilica via a co-condensation modification using caminopropyltriethoxysilane (APTES), Ceram. Int. 35 (2009) 1883–1888, https://doi.org/10.1016/j.ceramint.2008.10.028. [18] R.P. Bagwe, L.R. Hilliard, W. Tan, Surface modification of silica nanoparticles to reduce aggregation and nonspecific binding, Langmuir 22 (2006) 4357–4362, https://doi.org/10.1021/la052797j. [19] E. Che, L. Wan, Y. Zhang, Q. Zhao, X. Han, J. Li, J. Liu, S. Wang, Development of phosphonate-terminated magnetic mesoporous silica nanoparticles for pHcontrolled release of doxorubicin and improved tumor accumulation, Asian J. Pharm. Sci. 9 (2014) 317–323, https://doi.org/10.1016/j.ajps.2014.07.003. [20] E. Ozgur, P. Toren, M. Bayindir, Phosphonate based organosilane modification of a simultaneously protein resistant and bioconjugable silica surface, J. Mater. Chem. B 2 (2014) 7118–7122, https://doi.org/10.1039/C4TB01283F. [21] D.P. Ferris, J. Lu, C. Gothard, R. Yanes, C.R. Thomas, J.-C. Olsen, J.F. Stoddart, F. Tamanoi, J.I. Zink, Synthesis of biomolecule-modified mesoporous silica
[22]
[23]
[24]
[25]
[26]
9
nanoparticles for targeted hydrophobic drug delivery to cancer cells, Small 7 (2011) 1816–1826, https://doi.org/10.1002/smll.201002300. S.-M. Shang, Z. Li, Y. Xing, J.H. Xin, X.-M. Tao, Preparation of durable hydrophobic cellulose fabric from water glass and mixed organosilanes, Appl. Surf. Sci. 257 (2010) 1495–1499, https://doi.org/10.1016/j.apsusc.2010.08.081. D. Isßın, N. Kayaman-Apohan, A. Güngör, Preparation and characterization of UV-curable epoxy/silica nanocomposite coatings, Prog. Org. Coat. 65 (2009) 477–483, https://doi.org/10.1016/j.porgcoat.2009.04.007. N. Majoul, S. Aouida, B. Bessaïs, Progress of porous silicon APTESfunctionalization by FTIR investigations, Appl. Surf. Sci. 331 (2015) 388–391, https://doi.org/10.1016/j.apsusc.2015.01.107. G. Singh, N.H. Zaidi, U. Soni, M. Gautam, R. Jackeray, H. Singh, S. Sapra, Detection of bioconjugated quantum dots passivated with different ligands for bio-applications, J. Nanosci. Nanotechnol. 11 (2011) 3834–3842, https://doi. org/10.1166/jnn.2011.4169. R. Edgington, K.M. Spillane, G. Papageorgiou, W. Wray, H. Ishiwata, M. Labarca, S. Leal-Ortiz, G. Reid, M. Webb, J. Foord, N. Melosh, A.T. Schaefer, Functionalisation of detonation nanodiamond for monodispersed soluble DNA-nanodiamond conjugates using mixed silane bead-assisted sonication disintegration, Sci. rep. 8 (2018) 728, https://doi.org/10.1038/s41598-017-18601-6.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Application of surface-modified silica nanoparticles with dual silane coupling agents in bitumen for performance enhancement Sidharth Reddy Karnati a, Daniel Oldham b, Elham H. Fini b,⇑, Lifeng Zhang a,⇑ a b
Department of Nanoengineering, Joint School of Nanoscience and Nanoengineering, North Carolina A&T State University, Greensboro, NC 27401, USA School of Sustainable Engineering and Built Environment, Arizona State University, Tempe, AZ 85287, USA
h i g h l i g h t s The best result for SNP surface modification was from dual silane APTES-GPTMS. APTES-GPTMS-SNPs outperformed the reported APTES-SNPs for bitumen modification. The bitumen with APTES-GPTMS-SNPs had more resistance to aging, rutting & cracking. The enhanced bitumen was ascribed to better dispersion of APTES-GPTMS-SNPs. The result provides insight on bitumen enhancement particularly for delay of aging.
a r t i c l e
i n f o
Article history: Received 27 July 2019 Received in revised form 14 December 2019 Accepted 30 January 2020
Keywords: Silica nanoparticles Surface modification Silane coupling agent Dispersion Bitumen Anti-aging
a b s t r a c t Bitumen is a black and highly viscous liquid that holds stone aggregate together in road pavement. Oxidation aging of bitumen can accelerate overall pavement deterioration and shorten pavement service life. Use of silica nanoparticles (SNPs) have been promoted as a sustainable construction practice to delay bitumen oxidation. Nonetheless, adequate dispersion of SNPs in bitumen has been a challenge since their first employment. Surface functionalization of SNPs with (3-aminopropyl) triethoxysilane (APTES) has demonstrated its efficiency to improve dispersion of SNPs in bitumen at relatively high loading of SNPs with low mechanical energy input. As the amount of APTES increases in the process of surface modification of SNPs, however, average size of the resultant SNPs and their agglomeration in bitumen also increase due to APTES self-condensation reaction on SNP surface. This research investigated surface modification of SNPs with other silane coupling agents including 3-(trihydroxysilyl) propyl methylphosphonate (THPMP) and (3-glycidyloxypropyl) trimethoxysilane (GPTMS) as well as dual silane combinations: APTES with THPMP and APTES with GPTMS. The intention was to minimize agglomeration of SNPs in bitumen and to further improve overall performance of SNP-containing bitumen. The comparative experimental results indicated that the surface-modified SNPs with dual silanes (APTES-GPTMS) outperformed the sole APTES-modified SNPs regarding dispersion in bitumen, resulting in bitumen with more enhanced anti-aging and low temperature properties. The results of this study inform and promote the application of SNPs in road pavement with enhanced performance and sustainability. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, organic/inorganic nanomaterials such as carbon nanoparticles, silica nanoparticles, nanoclay, and carbon nanotubes have been used as modifiers of bitumen (sometimes termed as asphalt binder) to improve its performance [1–4]. Among all inorganic nanofillers, silica (SiO2) nanoparticles (SNPs) demonstrated great improvement in performance of bitumen including aging, ⇑ Corresponding authors. E-mail addresses: [email protected] (E.H. Fini), [email protected] (L. Zhang). https://doi.org/10.1016/j.conbuildmat.2020.118324 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
fatigue cracking, and rutting properties, but negatively impacted low temperature properties [5]. It is shown from previous studies that performance of the SNP-modified bitumen degraded when loading of SNPs went beyond 3 wt% even being mixed at high shearing speed (4000–5000 rpm), long mixing time (1–2 h) and high mixing temperature (> 135 °C) [2,5–9]. This degradation can be attributed to agglomeration of SNPs and their poor dispersion in bitumen at high loading. It has been well documented that surface modification of SNPs plays a vital role in improving their dispersion in polymer matrix as well as strengthening SNP-polymer matrix interface and has
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resulted in polymer nanocomposites with higher performance [10– 12]. (3-Aminopropyl) triethoxysilane (APTES) is a common silane coupling agent that is used for surface modification of SNPs and has been applied in different applications like polymer nanocomposites, drug delivery, and biosensing [13–15]. In our most recent research, APTES surface-modified SNPs (APTES-SNPs) was introduced to bitumen at 4% dosage by weight of bitumen [16]. Compared to the bitumen with pristine SNPs, the bitumen with APTES-SNPs allowed improvement in dispersion of SNPs in bitumen matrix and led to significant enhancement of bitumen’s resistance to aging, rutting as well as fatigue and low temperature cracking. It should be noted that as the amount of APTES increases in the process of surface modification of SNPs, average size of the resultant surface-modified SNPs and their agglomeration in bitumen increase due to APTES self-condensation reaction on SNP surface [17]. Large amount of APTES could also reduce stability of individual SNPs as indicated by average zeta potential of the surface modified SNPs [16]. This research aims to reduce agglomeration of surface-modified SNPs in bitumen by preventing self-condensation reaction of APTES through application of other silane coupling agents as well as through dual silane application. 3-(Trihydroxysilyl) propyl methylphosphonate (THPMP) and (3-Glycidyloxypropyl) trimethoxysilane (GPTMS) are two other common silane coupling agents. It is hypothesized that charge-bearing THPMP may help stabilize surface-modified SNPs through electrical repulsive force [18]. Considering that THPMP can make the surface of SNPs more hydrophilic [19,20] and GPTMS can make the surface of SNPs more hydrophobic [21–23], a comparative study on SNP surface modification and resultant bitumen performance may lead to a general guideline for tailoring surface modification of SNPs to be used in bitumen for road pavements. Specifically the effects of surface-modified SNPs on performance of bitumen were investigated and compared in this research. The surface modification of SNPs was confirmed by Fourier-transform infrared spectroscopy (FTIR). The morphology, size and stability of surface-modified SNPs were characterized using scanning electron microscope (SEM) and dynamic light scattering (DLS). Performance of bitumen with these surface-modified SNPs at 4 wt% loading was evaluated by aging, rutting, fatigue cracking, and low-temperature cracking. The relationship between surface modification of SNPs with different silanes/silane combinations and performance of the resultant modified bitumen was revealed. 2. Materials and methods 2.1. Materials The bitumen (PG 64-22, Table 1) in this research was acquired from Associated Asphalt in Greensboro, NC. (3-aminopropyl) triethoxysilane (APTES, catalog number A3648), 3-(trihydroxysilyl) propyl methylphosphonate (THPMP, catalog number 435716), (3glycidyloxypropyl) trimethoxysilane (GPTMS, catalog number 440167) and SiO2 nanoparticles (SNPs, catalog number 718483) with average size of 12 nm and 175–225 m2/g BET surface area
Table 1 General properties of bitumen PG 64-22. Specific Gravity @15.6 °C Flash Point, Cleveland Open Cup, °C Change in Mass RTFO Absolute Viscosity at 60 °C, Pas Stiffness (MPa) at 12 °C @ 60 s
1.039 335 0.0129 202 112.5
were purchased from Sigma Aldrich. Ethanol was purchased from Fisher Scientific. 2.2. Sample preparation 2.2.1. Surface modification of SNPs SNPs were first added in 95/5 (weight) ethanol/water mixture with stirring and then sonicated for 10 min to achieve good dispersion. To surface-modify pristine SNPs, individual silane coupling agent including APTES, THPMP, and GPTMS was added to the system, respectively, at a weight ratio of silane/SNP = 1.6 and sonicated for another 5 min. The mixture was further stirred for 2 h and maintained at 70 °C to obtain respective surface-modified SNPs with APTES (APTES-SNPs), with THPMP (THPMP-SNPs), and with GPTMS (GPTMS-SNPs). To further surface-modify APTESSNPs with GPTMS or THPMP (i.e. dual-silane modification), same surface-modification procedure as described above was followed except using APTES-SNPs as starting material. GPTMS or THPMP was added to the reaction system with APTES-SNPs and the whole system was stirred for another 2 h and maintained at 70 °C to obtain surface-modified SNPs with APTES and THPMP (APTESTHPMP-SNPs) and with APTES and GPTMS (APTES-GPTMS-SNPs), respectively. All the surface-modified SNPs including APTES-SNPs, THPMP-SNPs, GPTMS-SNPs, APTES-THPMP-SNPs, and APTESGPTMS-SNPs were individually separated by centrifugation, washed with fresh DI water thoroughly, and then freeze-dried for 24 h before application in bitumen. 2.2.2. Mixing SNPs with bitumen The bitumen was initially heated to 135 °C. Then 4 wt% of certain surface-modified SNPs were added and mixed with the bitumen using a mechanical stirring at 900 rpm for 30 min at 135 °C. 2.2.3. Aging of asphalt binder Aging of modified bitumen with respective silane-modified SNPs was performed through two stages: short-term aging and long-term aging. Rolling thin film oven (RTFO) procedure is used to perform short-term aging of unaged bitumen according to ASTM D 2872 at 163 °C for 85 min in the presence of constant air flow followed by long-term aging in a pressure aging vessel (PAV, Prentex Model 9300) based on ASTM D6521 at 100 °C under 300 psi for 20 h. 2.3. Characterization Surface modification of SNPs with silane coupling agents was characterized by a Varian 670 FTIR spectrometer. Hydrodynamic diameter and zeta potential of surface-modified SNPs were characterized by using dynamic light scattering (DLS, Malvern Zetasizer Nano ZS). For each SNP sample, 0.1 mg SNPs were sonicated for 5 min in 1 L of DI water and 1 ml of the water with SNPs was taken out afterwards to perform DLS measurement. A Zeiss Auriga Crossbeam FIB Field Emission Scanning Electron Microscope (FESEM) was used to characterize morphology of SNPs. A gold–palladium layer of 4 nm was sputter-coated on all SNP samples to improve image quality. A Zeiss Evo Environmental SEM with cool-stage was employed to characterize morphology and distribution of SNPs in bitumen. Characterization of bitumen samples including viscosity, aging and low temperature properties followed the same procedure as reported elsewhere [16]. Specifically, viscosity of bitumen samples was measured using a Brookfield DV-III ultra-rotational viscometer (RV) in accordance with ASTM D4402 at 135 °C with a speed of 20 rpm. Viscosity aging index (VAI) was calculated using viscosity of each bitumen sample before and after long-term aging. FTIR was used to monitor carbonyl and sulfoxide functional groups of
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unaged and aged bitumen samples to investigate oxidative aging behavior of bitumen with surface-modified SNPs. The IR absorbance peaks around 1700 cm 1 and 1030 cm 1 are attributed to be carbonyl and sulfoxide functional groups, respectively. Corresponding carbonyl or sulfoxide peak area in IR spectrum of a specific bitumen sample with surface-modified SNPs was divided by the entire area between 2000 and 600 cm 1 in the IR spectrum to calculate chemical aging indices (CAI) of both unaged and aged samples. A Kinexus dynamic shear rheometer (DSR-III, HM-87) was used to characterize both viscous and elastic behavior of bitumen samples by measuring complex shear modulus (G*) and phase angle (d) according to ASTM D7175-15. The DSR data for rutting resistance (G*/Sin d) evaluation were collected at 64 °C while the DSR data for fatigue cracking (G* Sin d) evaluation were collected at 28 °C. The rheological aging index (RAI) of each bitumen sample was determined using the results at 64 °C. The selected DSR frequency for these samples was 10 rad/s. Low temperature properties of bitumen samples with surface-modified SNPs including ductility and fracture energy were evaluated by a Direct Tension Tester (Interlaken) following ASTM 6723–12. The test was conducted at 12 °C with a displacement rate of 1.00 mm/min. Fracture energy was determined by the area under the load-displacement curve and ductility was determined based on elongation at break.
3. Results and discussion 3.1. Surface modification of SNPs with single and dual silanes FTIR was used to characterize surface modification of SNPs with THPMP, GPTMS as well as dual silanes (APTES + THPMP and APTES + GPTMS) (Fig. 1). In comparison with pristine SNPs, the IR spectrum of THPMP-SNPs exhibited peaks around ~2885 cm 1/ ~2940 cm 1 and ~1420 cm 1/~1350 cm 1, which are typical absorption peaks attributed to stretching and bending vibrations of C-H in –CH2– and –CH3 [24] and an evidence for successful attachment of THPMP on SNPs (Fig. 1a). The peak centered at ~1310 cm 1 is attributed to P = O stretching [25,26] and further confirmed the presence of THPMP on surface of SNPs [20] (Fig. 1a). Similarly, the IR spectrum of GPTMS-SNPs showed peaks at ~2935 cm 1/~2920 cm 1/~2900 cm 1/~2875 cm 1/~2840 cm 1 and ~1425 cm 1/~1360 cm 1, which are attributed to stretching and bending vibrations of C-H in –CH2– and –CH< in GPTMS molecules, indicating successful attachment of GPTMS on SNPs. The enhanced peak centered at ~810 cm 1 indicated the presence of C-O in epoxide functional groups. All of these observations confirmed the graft of GPTMS on surface of SNPs. FTIR spectra of pristine SNPs, APTES-SNPs, as well as dual-silane treated SNPs including APTES-THPMP-SNPs and APTES-GPTMSSNPs are compared in Fig. 1b. All the silane treated SNPs showed typical –CH3 and –CH2– peaks as described before. The FTIR spectrum of APTES-SNPs showed a peak centered at ~1590 cm 1 due to N–H bending in primary amine –NH2, indicating successful grafting of APTES on SNP surface [16]. Based on the FTIR spectrum of APTES-SNPs, it is observed that the characteristic peak of primary amine –NH2 disappeared in IR spectrum of APTES-THPMP-SNPs sample while the characteristic peak of P = O at ~1310 cm 1 was present, which indicated some reaction between APTES and THPMP on SNP surface. Since the graft of APTES on SNP surface was previously confirmed and THPMP was added after APTES functionalization, the presence of THPMP characteristic peaks suggested co-existence of APTES and THPMP on surface of SNPs. As for APTES-GPTMS-SNPs sample, the FTIR spectrum also showed disappearance of primary amine characteristic peak while an enhanced peak at ~1410 cm 1 could be assigned to O–H bending
3
in tertiary alcohol. This is a proof for the chemical reaction between APTES and GPTMS where epoxide rings in GPTMS molecules were opened by the amine functional groups in APTES, indicating APTES and GPTMS structures are connected and co-exist on SNP surface. The enhanced peaks in the region of ~3100 cm 1 and ~3500 cm 1 from FTIR spectra of APTES-THPMP-SNPs and APTESGPTMS-SNPs are attributed to –OH from THPMP and formation of tertiary alcohol due to epoxide ring opening from GPTMS, respectively, which is another evidence for the reactions between APTES and THPMP and between APTES and GPTMS, respectively. The possible chemical reaction mechanisms between SNPs and silane coupling agents are demonstrated in Fig. 2. To evaluate stability and size of various silane-treated SNPs, average size and zeta potential of all surface modified SNPs were characterized by using DLS and compared with those of APTESSNPs (Fig. 3). The average size and zeta potential of APTES-SNPs were 269 nm & 11.5 mV. Compared to APTES, employment of THPMP increased average size of the resultant surface-modified SNPs by 36% and reduced average zeta potential of the SNPs by 14% while grafting GPTMS on SNPs increased average size of the resultant surface-modified SNP by 11% and increased average zeta potential of the SNPs by 152%. In the case of dual silanes, compared to APTES, APTES-THPMP increased average size of the reultant surface-modified SNPs by 9% and increased average zeta potential of the SNPs by 48% while APTES-GPTMS reduced average size of the resultant surface-modified SNPs by 19% and increased average zeta potential of the SNPs by 123%. From the point of view of stability (larger absolute value of zeta potential) and less agglomeration (smaller avearge size), the dual silanes with APTES and GPTMS demonstrated the best overall result followed by GPTMS alone. In other words, GPTMS can be used to replace APTES to modify pristine SNPs or further modify APTES-SNPs for better particle stability and smaller particle size. Compared to the case of APTES where positive ions (–NH+3) and negative ions (–Si-O-) together determines overall zeta potential of the SNPs, the carbon atoms in epoxide groups of GPTMS are electrophiles and can attract negative charges. In combination with the negative ions of –Si-O- on surface of SNPs, GPTMS-SNPs exhibited the overall largest negative charges, which has the greatest potential to facilitate dispersion and stabilization of SNPs in bitumen. 3.2. Comparative performance evaluation of bitumen samples with silane surface-modified SNPs 3.2.1. Aging evaluation Surface-modified SNPs were individually added to respective bitumen samples at 4 wt% loading and their effects on aging of the bitumen were examined using viscosity aging index (VAI), chemical aging index (CAI) including carbonyl index and sulfoxide index, and rheological aging index (RAI) (Fig. 4). Viscosity of the bitumen PG 64–22 containing 4 wt% APTESSNPs was found to be 36% and 63% less than that of the bitumen containing the same amount of pristine SNPs before and after aging [16]. Compared to APTES-SNPs, employment of THPMP-SNPs, GPTMS-SNPs, and APTES-THPMP-SNPs reduced the bitumen’s viscosity before aging by 7%, 2%, and 12%, respectively, while APTES-GPTMS-SNPs increased the bitumen’s viscosity before aging by nearly 22%. Compared to the bitumen with APTES-SNPs, viscosities of bitumen samples after aging increased by 2% and 7% with THPMP-SNPs and APTES-THPMP-SNPs, respectively, but reduced by 11% and 24%, respectively, with GPTMS-SNPs and APTESGPTMS-SNPs. Compared to APTES-SNPs, THPMP-SNPs and APTES-THPMPSNPs increased VAI of the bitumen by 12% and 29%, respectively, while GPTMS-SNPs and APTES-GPTMS-SNPs reduced it by 13% and 51%, respectively. As evidenced by better particle stability
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Fig. 1. FTIR spectra of pristine SNPs and surface-modified SNPs with THPMP, GPTMS, and APTES based dual silanes.
and smaller particle size (Fig. 3), GPTMS-containing SNPs could have better distribution in bitumen matrix and thus have more interaction between molecules in bitumen and surface-modified SNPs, reducing internal friction between bitumen molecules (lowering the viscosity) and chance of reaction between bitumen and oxygen (more aging resistance). On the contrary, THPMPcontaining SNPs worsened the oxidative aging of the bitumen due to their relatively poor distribution in bitumen matrix caused by larger particle size and lower zeta potential as well as reduced compatibility between THPMP-SNPs and overall hydrophobic bitumen based on more hydrophilicity of THPMP than APTES. Compared to the bitumen with APTES-SNPs, the bitumen containing THPMP-SNPs and APTES-THPMP-SNPs showed 24% and
26% more carbonyl CAI and 452% and 375% more sulfoxide CAI, respectively, while the bitumen with GPTMS-SNPs and APTESGPTMS-SNPs reached 31% and 23% less carbonyl CAI but 289% and 105% more sulfoxide CAI, respectively. The smallest sulfoxide CAI thus occurred with APTES. Interactions such as hydrogen bonding and/or dipole-dipole interaction between APTES molecules on APTES-SNPs and molecules in bitumen could reduce chance of oxidative reaction between the bitumen molecules and environmental oxygen, thus lowering amount of carbonyl and sulfoxide functional groups during oxidation and reducing CAI [16]. GPTMS-containing SNPs showed reduction in carbonyl CAI compared to APTES-SNPs possibly due to more oxygen atoms in GPTMS molecular structure for hydrogen bonding and/or dipole-dipole interaction with bitumen molecules. It is noteworthy that sulfur
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Fig. 2. Schematic illustration of possible reactions between SNPs and silane coupling agents.
has stronger electronegativity than carbon while sulfoxide is more polar than carbonyl group. According to chemical structures of the silanes in this research (Fig. 2), there were more chances for –NH2 in APTES than epoxide functional group in GPTMS to form hydrogen bonding and/or dipole-dipole interaction with sulfurcontaining molecules in bitumen. The THPMP-containing SNPs may be able to form hydrogen bonding and/or dipole-dipole interaction with polar groups in bitumen as well, but their relatively bigger size and less homogeneous distribution in bitumen could counteract the abovementioned interaction and thus lead to increased carbonyl CAI and much larger sulfoxide CAI. RAI characterizes oxidative aging of bitumen based on nonrecoverable deformation and hardening. Compared to the bitumen sample with APTES-SNPs, the bitumen samples with THPMP-SNPs and APTES-THPMP-SNPs exhibited 25% and 32% increase in RAI, respectively, while the bitumen samples with GPTMS-SNPs and APTES-GPTMS-SNPs presented 16% reduction and 2% increase in RAI, respectively. The RAI results further demonstrated that THPMP is not a good surface-modifying agent for SNPs from anti-
aging perspective while GPTMS performed better compared to APTES. 3.2.2. Rutting resistance, fatigue cracking and low temperature cracking properties Combination of complex shear modulus (G*) and phase angle (d) from DSR measurement was used to characterize rutting resistance (G*/Sin d) and fatigue cracking (G* Sin d) of bitumen samples containing surface-modified SNPs with single and dual silanes. Higher value of G*/Sin d indicates higher resistance to permanent deformation (rutting) in asphalt pavement. Lower value of G* Sin d suggests higher resistance to fatigue cracking in asphalt pavement. Experimental results (Fig. 5) revealed that only the bitumen sample containing APTES-GPTMS-SNPs had higher rutting resistance than that of the one containing APTES-SNPs, which reflected a 7% improvement. Rutting resistance of the bitumen samples containing THPMP-SNPs, GPTMS-SNPs, APTES-THPMPSNPs decreased by 10%, 12%, and 17%, respectively, compared to that of the bitumen containing APTES-SNPs.
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500
Average size (nm) Average Zeta Potential (mV)
450 400 350 300 250 200 150 100 50 0 -50
APTES
THPMP
GPTMS
APTES
APTES
THPMP
GPTMS
SNPs Fig. 3. Average size and average zeta potential of surface-modified SNPs with single and dual silane coupling agents.
Fatigue cracking of bitumen samples containing GPTMSSNPs and APTES-GPTMS-SNPs was reduced by 27% and 33%, respectively, compared to that of the bitumen sample containing APTES-SNPs. However, fatigue cracking of the bitumen samples containing THPMP-SNPs and APTES-THPMP-SNPs increased by 16% and 12%, respectively. From the point of view of rutting resistance and reduction of fatigue cracking, APTESGPTMS-SNPs performed the best among all scenarios studied here. Low temperature performance of unaged and aged bitumen samples with APTES-SNPs, GPTMS-SNPs and APTES-GPTMSSNPs were further characterized by measuring their fracture energy and ductility (Fig. 6). As for unaged samples, the bitumen samples with GPTMS-SNPs and APTES-GPTMS-SNPs exhibited a reduction of 10% and 5% in fracture energy, respectively, compared to the bitumen sample containing APTES-SNPs. However, fracture energy of bitumen samples with APTES-GPTMS-SNPs and GPTMS-SNPs after aging was found to be 5% higher and 60% lower, respectively, than that of the bitumen sample containing APTES-SNPs. Compared to the sample containing APTES-SNPs, the bitumen sample with APTES-GPTMS-SNPs showed similar ductility while the bitumen sample with
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Fig. 4. Aging characterization of bitumen samples containing 4 wt% surface-modified SNPs with single and dual silanes: (a) viscosities before and after long-term aging; (b) long-term viscosity aging indices; (c) carbonyl and sulfoxide chemical aging indices; (d) rheological aging indices.
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Fatigue Cracking G*× Sin δ (kPa)
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Fig. 5. Rutting resistance and fatigue cracking of bitumen samples containing 4 wt% surface-modified SNPs with single and dual silanes.
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Fig. 6. Fracture toughness and ductility of unaged and aged bitumen samples containing 4 wt% surface-modified SNPs with single and dual silane coupling agents.
GPTMS-SNPs presented lower ductility for both unaged and aged samples: 15% reduction for the unaged sample and 41% reduction for the aged sample. Overall, the aged bitumen containing APTES-GPTMS-SNPs slightly outperformed the aged bitumen containing APTES-SNPs in low temperature properties. Based on all the experimental data herein, performance of silane-modified SNPs in bitumen can be ascribed to two factors: (1) SNP distribution in bitumen matrix; (2) interfacial strength between SNP and bitumen matrix. Stronger interfacial bonding between SNPs and bitumen matrix requires more energy to undergo de-bonding and thus improve mechanical properties of bitumen as indicated in polymer nanocomposite [13]. According to SNP morphology from SEM, GPTMS-SNPs and APTES-GPTMSSNPs showed much smaller size and more homogeneous distribution in bitumen matrix than APTES-SNPs (Fig. 7). APTESGPTMS-SNPs performed overall the best among all silane surfacemodified SNPs in this research and particularly outperformed APTES-SNPs, which was investigated in previous study and demon-
strated their effectiveness in property enhancement of bitumen [16]. The observed bitumen property improvement from APTESGPTMS-SNPs can be attributed to their better dispersion in bitumen matrix than APTES-SNPs due to the break of APTES selfcondensation reaction through GPTMS reaction with APTES. The dispersion of APTES-GPTMS-SNPs in bitumen matrix could be further facilitated by enhanced compatibility of the SNPs with bitumen due to more hydrophobic nature of the SNP surface from reaction between APTES and GPTMS than that with only APTES. Compared to APTES-SNPs, the improvement in compatibility of APTES-GPTMS-SNPs with bitumen not only ensured enough chance to form hydrogen bonding and/or dipole–dipole interaction with polar groups in bitumen to reduce chance of reaction between molecules in bitumen and environmental oxygen, but also simultaneously provided better interfacial bonding between SNPs and bitumen matrix. Consequently, the dual silane (APTES-GPTMS) surface modified SNPs demonstrated more enhanced anti-aging resistance and low temperature properties.
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Fig. 7. SEM images showing distribution of APTES-SNPs (a), GPTMS-SNPs (b), and APTES-GPTMS-SNPs (c) in bitumen.
4. Conclusion
Acknowledgement
Surface modification of silica nanoparticles (SNPs) with silane coupling agents including 3-(trihydroxysilyl) propyl methylphosphonate (THPMP), (3-glycidyloxypropyl) trimethoxysilane (GPTMS), and dual silane combinations of (3-aminopropyl) triethoxysilane (APTES) with THPMP and APTES with GPTMS was successfully carried out. Surface-modified SNPs were then introduced to bitumen (PG 64-22) at 4 wt% loading under relatively low mixing temperature (135 °C), low shear rate (900 rpm), and short mixing time (30 min). Among all the investigated silanes and silane combinations, the combination of APTES with GPTMS demonstrated the best result from the points of view of stability, agglomeration, distribution in bitumen, and property enhancement of bitumen. APTES-GPTMSSNPs outperformed the previously reported APTES-SNPs for application in bitumen and rendered bitumen more resistance to aging, rutting, fatigue cracking and low temperature cracking. Compared to APTES-SNPs, specifically, APTES-GPTMS-SNPs reduced particle average size by 19% and increased particle average zeta potential by 123%. Further comparison revealed that viscosity aging index (VAI) and carbonyl chemical aging index (CAI) of the bitumen was lowered by 51% and by 23%, respectively, when APTES-GPTMSSNPs was employed instead of APTES-SNPs. In addition, the resistance to rutting, low temperature cracking, and fatigue cracking was 7%, 5%, and 33%, respectively, higher for the bitumen sample with APTES-GPTMS-SNPs than the one with APTES-SNPs. Compared to APTES-SNPs, the observed bitumen property improvements from APTES-GPTMS-SNPs implementation was attributed to better dispersion of this dual-silane treated SNPs, which was mainly resulted from the reduction of APTES selfcondensation reaction through the reaction between GPTMS and APTES as well as increased compatibility between APTES-GPTMSSNPs and bitumen due to more hydrophobic nature of APTESGPTMS-SNPs with respect to APTES-SNPs. It is suggested that surface-modification of SNPs can be judiciously adjusted to maximize the enhancement of bitumen for road pavement.
The authors thank the financial support from the Joint School of Nanoscience and Nanoengineering of North Carolina A&T State University, a member of Southeastern Nanotechnology Infrastructure Corridor (SENIC) and National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (ECCS-1542174).
CRediT authorship contribution statement Sidharth Reddy Karnati: Investigation, Formal analysis, Writing - original draft. Daniel Oldham: Investigation, Data curation, Validation. Elham H. Fini: Conceptualization, Methodology, Writing - review & editing. Lifeng Zhang: Conceptualization, Resources, Writing - review & editing, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
References [1] F. Xiao, A.N. Amirkhanian, S.N. Amirkhanian, Influence of carbon nanoparticles on the rheological characteristics of short-term aged asphalt binders, J. Mater. Civ. Eng. 23 (2011) 423–431, https://doi.org/10.1061/(ASCE)MT.19435533.0000184. [2] E.H. Fini, P. Hajikarimi, R. Mohammad, F.M. Nejad, Physiochemical, rheological, and oxidative aging characteristics of asphalt binder in the presence of mesoporous silica nanoparticles, J. Mater. Civ. Eng. 28 (2016) 04015133, https://doi.org/10.1061/(ASCE)MT.1943-5533.0001423. [3] M. Abdelrahman, D.R. Katti, A. Ghavibazoo, H.B. Upadhyay, K.S. Katti, Engineering physical properties of asphalt binders through nanoclay–asphalt interactions, J. Mater. Civ. Eng. 26 (2014) 04014099, https://doi.org/10.1061/ (ASCE)MT.1943-5533.0001017. [4] M. Faramarzi, M. Arabani, A.K. Haghi, V. Mottaghitalab, Carbon nanotubesmodified asphalt binder: preparation and characterization, Int. J. Pavement Res. Technol. 8 (2015) 29–37, https://doi.org/10.6135/ijprt.org.tw/2015.8 (1).29. [5] H. Yao, Z. You, L. Li, C.H. Lee, D. Wingard, Y.K. Yap, X. Shi, S.W. Goh, Rheological properties and chemical bonding of asphalt modified with nanosilica, J. Mater. Civ. Eng. 25 (2013) 1619–1630, https://doi.org/10.1061/(ASCE)MT.19435533.0000690. [6] Z. You, J. Mills-Beale, J.M. Foley, S. Roy, G.M. Odegard, Q. Dai, S.W. Goh, Nanoclay-modified asphalt materials: preparation and characterization, Constr. Build. Mater. 25 (2011) 1072–1078, https://doi.org/10.1016/ j.conbuildmat.2010.06.070. [7] N.I.M. Yusoff, A.A.S. Breem, H.N.M. Alattug, A. Hamim, J. Ahmad, The effects of moisture susceptibility and ageing conditions on nano-silica/polymermodified asphalt mixtures, Constr. Build. Mater. 72 (2014) 139–147, https:// doi.org/10.1016/j.conbuildmat.2014.09.014. [8] G.M. Amin, A. Esmail, Application of nano silica to improve self-healing of asphalt mixes, J. Cent. South Univ. 24 (2017) 1019–1026, https://doi.org/ 10.1007/s11771-017-3504-y. [9] X. Guo, M. Sun, W. Dai, S. Chen, Performance characteristics of silane silica modified asphalt, Adv. Mater. Sci. Eng. 2016 (2016) 6731232, https://doi.org/ 10.1155/2016/6731232. [10] M. Zamanian, M. Mortezaei, B. Salehnia, J.E. Jam, Fracture toughness of epoxy polymer modified with nanosilica particles: Particle size effect, Eng. Fract. Mech. 97 (2013) 193–206, https://doi.org/10.1016/j.engfracmech.2012.10.027. [11] M.Z. Rong, M.Q. Zhang, W.H. Ruan, Surface modification of nanoscale fillers for improving properties of polymer nanocomposites: a review, Mater. Sci. Technol. 22 (2006) 787–796, https://doi.org/10.1179/174328406X101247. [12] A. Hirsch, Functionalization of single-walled carbon nanotubes, Angew. Chem. Int. Ed. 41 (2002) 1853–1859, https://doi.org/10.1002/1521-3773(20020603) 41:11<1853::AID-ANIE1853>3.0.CO;2-N. [13] G. Wang, D. Yu, R.V. Mohan, S. Gbewonyo, L. Zhang, A comparative study of nanoscale glass filler reinforced epoxy composites: electrospun nanofiber vs nanoparticle, Compos. Sci. Technol. 129 (2016) 19–29, https://doi.org/ 10.1016/j.compscitech.2016.04.006. [14] F. Tang, L. Li, D. Chen, Mesoporous silica nanoparticles: synthesis, biocompatibility and drug delivery, Adv. Mater. 24 (2012) 1504–1534, https://doi.org/10.1002/adma.201104763. [15] K. De Vos, I. Bartolozzi, E. Schacht, P. Bienstman, R. Baets, Silicon-on-Insulator microring resonator for sensitive and label-free biosensing, Opt. Express 15 (2007) 7610–7615, https://doi.org/10.1364/OE.15.007610.
S.R. Karnati et al. / Construction and Building Materials 244 (2020) 118324 [16] S.R. Karnati, D. Oldham, E.H. Fini, L. Zhang, Surface functionalization of silica nanoparticles to enhance aging resistance of asphalt binder, Constr. Build. Mater. 211 (2019) 1065–1072, https://doi.org/10.1016/ j.conbuildmat.2019.03.257. [17] I.A. Rahman, M. Jafarzadeh, C.S. Sipaut, Synthesis of organo-functionalized nanosilica via a co-condensation modification using caminopropyltriethoxysilane (APTES), Ceram. Int. 35 (2009) 1883–1888, https://doi.org/10.1016/j.ceramint.2008.10.028. [18] R.P. Bagwe, L.R. Hilliard, W. Tan, Surface modification of silica nanoparticles to reduce aggregation and nonspecific binding, Langmuir 22 (2006) 4357–4362, https://doi.org/10.1021/la052797j. [19] E. Che, L. Wan, Y. Zhang, Q. Zhao, X. Han, J. Li, J. Liu, S. Wang, Development of phosphonate-terminated magnetic mesoporous silica nanoparticles for pHcontrolled release of doxorubicin and improved tumor accumulation, Asian J. Pharm. Sci. 9 (2014) 317–323, https://doi.org/10.1016/j.ajps.2014.07.003. [20] E. Ozgur, P. Toren, M. Bayindir, Phosphonate based organosilane modification of a simultaneously protein resistant and bioconjugable silica surface, J. Mater. Chem. B 2 (2014) 7118–7122, https://doi.org/10.1039/C4TB01283F. [21] D.P. Ferris, J. Lu, C. Gothard, R. Yanes, C.R. Thomas, J.-C. Olsen, J.F. Stoddart, F. Tamanoi, J.I. Zink, Synthesis of biomolecule-modified mesoporous silica
[22]
[23]
[24]
[25]
[26]
9
nanoparticles for targeted hydrophobic drug delivery to cancer cells, Small 7 (2011) 1816–1826, https://doi.org/10.1002/smll.201002300. S.-M. Shang, Z. Li, Y. Xing, J.H. Xin, X.-M. Tao, Preparation of durable hydrophobic cellulose fabric from water glass and mixed organosilanes, Appl. Surf. Sci. 257 (2010) 1495–1499, https://doi.org/10.1016/j.apsusc.2010.08.081. D. Isßın, N. Kayaman-Apohan, A. Güngör, Preparation and characterization of UV-curable epoxy/silica nanocomposite coatings, Prog. Org. Coat. 65 (2009) 477–483, https://doi.org/10.1016/j.porgcoat.2009.04.007. N. Majoul, S. Aouida, B. Bessaïs, Progress of porous silicon APTESfunctionalization by FTIR investigations, Appl. Surf. Sci. 331 (2015) 388–391, https://doi.org/10.1016/j.apsusc.2015.01.107. G. Singh, N.H. Zaidi, U. Soni, M. Gautam, R. Jackeray, H. Singh, S. Sapra, Detection of bioconjugated quantum dots passivated with different ligands for bio-applications, J. Nanosci. Nanotechnol. 11 (2011) 3834–3842, https://doi. org/10.1166/jnn.2011.4169. R. Edgington, K.M. Spillane, G. Papageorgiou, W. Wray, H. Ishiwata, M. Labarca, S. Leal-Ortiz, G. Reid, M. Webb, J. Foord, N. Melosh, A.T. Schaefer, Functionalisation of detonation nanodiamond for monodispersed soluble DNA-nanodiamond conjugates using mixed silane bead-assisted sonication disintegration, Sci. rep. 8 (2018) 728, https://doi.org/10.1038/s41598-017-18601-6.