Stimulated Adsorption of Humic Acids on Capped Plasmonic Ag

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Stimulated Adsorption of Humic Acids on Capped Plasmonic Ag Nanoparticles Investigated by Surface-Enhanced Optical Techniques Ornella Francioso, Eduardo Lopez-Tobar, Armida Torreggiani, Mercedes Iriarte, and Santiago Sanchez-Cortes Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00190 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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Stimulated Adsorption of Humic Acids on Capped Plasmonic Ag Nanoparticles Investigated by Surface-Enhanced Optical Techniques

Ornella Francioso1, Eduardo López-Tobar2, Armida Torreggiani3 and Santiago Sanchez-Cortes2. 1

2

3

Dipartimento di Scienze Agrarie, Università di Bologna, Bologna, Italy

Instituto de Estructura de la Materia, IEM-CSIC, Serano 121, 28006, Madrid, Spain

ISOF. Consiglio Nazionale delle Ricerche, Via P. Gobetti 101, 40129 Bologna, Italy

*Corresponding author

S.Sanchez-Cortes Current address: Instituto de Estructura de la Materia, IEM-CSIC Serrano 121 28006, Madrid, Spain Fax: +34 91 564 5557

Phone: + 34 91 561 6800

E-mail: [email protected]

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Abstract

The adsorption of humic substances on Ag nanoparticles (AgNPs) is of a crucial environmental importance and determines the toxicity of these NPs and the structure of adsorbed organic matter. In this work, the adsorption of two standard soil and leonardate IHSS (International Humic Substances Society) humic acids was studied on Ag nanoparticles (AgNPs) of different size, shapes (spherical and star-like) and interfacial chemical composition. Surface-enhanced optical (Raman and fluorescence) spectroscopies were used to follow the specific chemical groups involved in this adsorption. By means of the latter optical techniques, information regarding the binding mechanism and the macromolecular aggregation can be deduced. The influence of the surface chemical composition induced by the different functionalization of the interfaces of these nanoparticles is highly important regarding the chemical interactions of these complex organic macromolecules. The surface functionalization with positive charged alkyl diamines led to a large increase in the adsorption as well as a strong structural rearrangement of the macromolecule once adsorbed onto the surface.

Keywords: SERS, SEF, Humic acids, Silver Nanoparticles, Functionalization


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1. Introduction

Metal nanoparticles (MNPs) are being increasingly produced due the strong demand for nanotechnological devices that can be applied in a wide range of activities: medicine, electronics, environment, energy production1. In particular, there has been an increasing interest in the environmental impact of silver nanoparticles (AgNPs) as demonstrated by the exponential growth of works published on these systems2. AgNPs are released into the environment during the lifecycle of consumer products since these nanoparticles can be used as antibacterial agents3 in products like clothes, cosmetic products, food storage containers, household appliances or children’s toys4. The consequences of this release are unpredictable. It is currently assumed that AgNPs have a high impact on the environment at different levels. For instance, the high cytotoxicity of AgNPs has been reported after internalization inside the cell5,6. At a broader scale there is also evidence of the high ecotoxicity of the release of AgNPs, which can affect the growth and reproduction of many species leading to modifications in the community composition, decrease of biomass and changes in the community activities7. Regarding the fate of AgNPs, it was reported that AgNPs undergo a strong degradation leading to Ag+ and finally AgS2 by the action of oxygen8. The presence in the environment of natural organic matter (NOM) is of great importance in relation to the dissolution and stability of AgNPs 9. This has a beneficial effect on the environment since the organic matter helps to avoid the regeneration and coprecipitation of AgNPs, promoting their disappearance from the environment and reducing the toxicity towards organisms. Therefore, the adsorption and interaction of NOM with AgNPs, and mineral nanoparticles in general, is of a great importance in many aspects of the environment since: a) NOM stabilizes MNPs in solution contributing to their sequestration, b) the structure of NOM can be modified upon its adsorption on a mineral matrix10,11; c) this adsorption regulates the speciation and mobility of the contaminants existing in the soil12; d) the adsorbed organic matter determines the chemical properties of the system13; finally, e) the metal functionalization with NOM can be employed in the construction of sensing devices for pollutant detection14. Apart from the fact that AgNPs are commonly found in the environment and can interact with NOM, AgNPs capped with different molecules can be employed as a mimetic system to simulate the interaction of the organic matter with naturally existing particles in soils or in aqueous suspension. In this sense, the functionalization of Ag surfaces with both anionic and cationic species


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determines in a large extent the adsorption capability of NOM and the aggregation/disggregation of natural macromolecules15. The interaction of NOM with mineral surfaces was previously studied on several matrices revealing that it depends on factors such as the pH, the ionic strength and the surface potential 12,16 . Additionally, the adsorption mechanism can be modulated by the presence of different molecular functionalization on the surface of nanoparticles, and this was previously demonstrated by studying their aggregation and the zeta potential16. Humic acids (HA) are a fraction of NOM, which are widely distributed in soil and water and represent the largest pool of recalcitrant organic carbon in the terrestrial environment17. The variety and complexity of chemical and biological reactions involved in their formation make HA a very hot topic in soil organic chemistry. Even though some components, such as complex mixtures of microbial and plant biopolymers and their degradation products have been identified, no distinct chemical category has been recognized18. As proposed by several authors, HA may be associated to supramolecular structures whose properties are determined by electrostatic, hydrogen bonding, and weak intermolecular interactions19,20,21. Therefore, the study of the shape, structural molecular rearrangement, and composition of HA represents a focal point in understanding their physico-chemical reactions and especially in predicting carbon sequestration and climate change. Surface-Enhanced Vibrational Spectroscopy was reported to be an excellent tool in the analysis of the molecular structure of adsorbates on metal surfaces22. Thanks to the use of nanostructured metals, the intrinsic weakness of Raman scattering can be overcome. This is possible by the localized surface plasmon resonance (LSPR) effect occurring on AgNPs of certain sizes and shapes23 using plasmonic nanoparticles of silver, which enhances the electric field on the surface leading to Surface-Enhanced Optical Spectroscopy, in particular Surface-Enhanced Raman Scattering (SERS) and Surface-Enhanced Fluorescence (SEF) spectroscopy22. The short range character of the LSPR effect can be used to investigate the chemical groups that directly interact with the surface, which may change depending on the structure of humic acids in solution. On the other hand, this structure can be changed at different pH 24,25,26. The fluorescence emission is usually quenched on AgNPs by energy transfer between the excited state of the adsorbed organic molecule and the metal. However, in the case of humic acids (HA) fluorophore groups are in the inner part of the macromolecule, i.e. far away from the metal surface quenching area, and then it is possible to have an intensification of the fluorescence due to the LSPR effect24. Therefore, in such cases the study of fluorescence can be very useful in the ACS Paragon Plus Environment

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structural treatment of humic acids and processes that can affect this structure like molecular folding and aggregation processes undergone by these important NOMs in solution24. In previous works we have investigated the interfacial properties of peat humic and fulvic acids, extracted with pyrophosphate, on metal nanoparticles27,28,29. In this work we investigated the adsorption of two standard soil and leonardate IHSS (International Humic Substances Society) on AgNPs of different nature by surface–enhanced optical spectroscopy, specifically SERS and SEF. These macromolecules were employed in order to correlate the dynamic behavior of these two subclasses with their specific structure. In the present work we have pay attention on the effect of the nanoparticle functionalization on the adsorption of these important environmental substances. To accomplish this, spherical AgNPs prepared by reduction with citrate or hydroxylamine, and starshaped AgNPs were obtained by a combined use of hydroylamine/citrate reducers. Using these methods AgNPs capped with different negative charges (citrate, chloride) can be obtained. Thanks to this, it is possible to study the pH effect on the HA adsorption, since the protonation of key structural groups such as carboxylic and phenolic groups can modify the supramolecular conformation of HA11,20,21. In fact, the chemical composition of the interface plays a crucial role in the adsorption of NOM on an inorganic surface by changing the surface electric potential afforded by the electric charge16, 30 . In addition, the chemical species existing on the interface can modulate the adsorption of HA by promoting chemical interactions through H-bonds. The adsorption of these macromolecules on the nanoparticles was further enhanced by using alkyl diamines as functionalizing molecules on the surface (Fig. 1A). In a previous study, positively charged aromatic molecules were demonstrated to be able to modify the interfacial properties of metal NPs favoring the adsorption of humic substances25. In particular, aliphatic α,ω-diamines (NH3+-(CH2)n-NH3+) of different lengths (n = 2,6,8,10,12) were selected for the MNP functionalization in order to reverse the negative residual charge of MNPs towards positive values so that the affinity of HA for the negative surface of AgNPs could be enhanced.

Materials and Methods Reagents AgNO3, trisodium citrate (CT) and hydroxilamine hydrochloride were purchased from Sigma Aldrich (Sant Louis, USA). The aqueous solutions were prepared by using milliQ water. 1.2Diaminoethane (AD2) and 1.12-diaminododecane (AD12) were purchased from Aldrich, whereas 1.6-Diaminohexane (AD6), 1.8-diaminooctane (AD8) and 1.10-diaminodecane (AD10) were


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purchased from Fluka, all with a purity of > 98% w/w. Aqueous stock solutions of AD2, AD6 and AD8 were prepared in Milli-Q water. AD10 and AD12 were dissolved in absolute ethanol. Leonardate humic acid Std (code1S104H) and Elliott Soil Humic Acid Std (code 1S102H) (Saint Paul, MN, USA) were purchased from the International Humic Substances Society (IHSS). Leonardate humic acid and Elliott Soil Humic Acid were named LHA and SHA, respectively. The different pH values were obtained by adding 0.1 N NaOH or 0.1 N HCl to the HA solutions.

Preparation of Ag nanoparticles Citrate Ag NPs (AgCT) were prepared by the following procedure. A total of 1 mL of a 1% (w/v) trisodium citrate (CT) aqueous solution was added to 50 mL of a boiling 10-3 M silver nitrate aqueous solution, and boiling was continued for 1 h. The obtained colloidal suspension showed a turbid gray aspect and had a final pH of 6.5. Hydroxylamine Ag NPs (AgHX) were obtained by the method described previously31. A total of 300 μL of NaOH (1 M) was added to 90 mL of a 6 × 10−2 M hydroxylamine hydrochloride solution. Then, 10 mL of a 1.1 × 10−3 M silver nitrate aqueous solution was added dropwise to the mixture under vigorous stirring. Finally, a brown silver colloid was obtained with pH 5.5. All processes were carried out at room temperature. The silver sol prepared was aged 24 h before experiments. Star-shaped nanoparticles (AgNS) were prepared by chemical reduction of Ag+ in two steps and using as reducing agents neutral hydroxylamine (HX) in a first step and CT in a second step32. 500 μL of 6 × 10-2 M HA were mixed with 500 μL of NaOH (0.05 M). Afterward, 9 mL of 10-3 M AgNO3 were added dropwise to the first solution under agitation. The suspension became brown. After 5 min, 100 μL of 4.13 × 10-2 M (1%, w/v) trisodium citrate were added to the mixture. The final suspension was shaken for 15 min showing a dark gray color. Samples preparation for the spectroscopic analysis The mother solution of humic acids (HA) was prepared by dissolving 1 mg HA in 1mL of NaOH 1M. Then, aliquots of different volumes were added to the corresponding AgNPs in suspension to reach the desired concentration. The final mixture was activated in order to make HA detectable by SERS; this was done by adding 10 µL of 0.5 M KNO3, which induced the subsequent aggregation of NPs and an increase in the number of hot spots where the electric field is further enhanced. Functionalization of AgNP with AD was carried out by adding an aliquot of aqueous


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chloride solution to a final concentration of 2x10-2 M and, subsequently, by adding aliquots of AD solutions up to the desired concentration. Instrumentation UV-Vis absorption spectra of colloids were recorded from 400 nm to 800 nm, on a Shimadzu 3600 spectrometer by using quartz cells, 1 cm optical path. Samples were diluted 30% in MilliQ water (volume/volume). SERS and SEF spectra were collected with a Renishaw Raman Invia Spectrometer (Renishaw Iberica S.A.U., Gavá, Spain), equipped with a Leica microscope, and an electrically refrigerated CCD camera, using the 532 (Nd:YAG) excitation line and a power of 2 mW. The SERS spectra were the result of one scan registered with an integration time of 10 s and a resolution of 2 cm-1. Transmission Electron Microscopy (TEM) images were obtained with an Inspect S50 (FEI, Hillsboro, Oregon, OH, USA) scanning electron microscope. The analysis was performed with 20kV as High Voltage. Extinction spectra of colloids were recorded on a Shimadzu 3600 spectrometer (Shimadzu Corp., Kyoto, Japan) equipped with a PMT for light detection in the UVvisible range and an InGaAs detector for the NIR was employed to obtain the plasmon extinction spectra. Samples were placed in quartz cells of 1 cm optical path, after dilution to 30% in MilliQ water (v/v). Results and Discussion Characterization of AgNPs The UV-vis spectra of metal NPs used in this work are shown in Fig. 1B, as well as the TEM images of the corresponding NPs (Fig. 1C). These nanoparticles display maxima at 400 (AgHX), 420 (AgCT) and 379/421 nm (AgNS). The peak position and the broadness of the extinction spectrum depend on the size and shape of the NPs. While AgHX shows a narrower band, due to a more homogeneous size distribution, the citrate one is broader due to the existence of oblates and small rods. AgNS is integrated by larger nanoparticles with different arms displaying an additional extinction in the red-NIR (Near Infrared) regions. Raman spectra of AgNPs before the adsorption of HA were recorded in order to study the final chemical species existing in the corresponding interfaces and which will influence the adsorption of HA on the surface. The blank Raman spectra of MNPs registered at pH 5.5 are shown in Figure S1 (Supporting Information). AgCT nanoparticles are characterized by strong bands corresponding to citrate ions adsorbed on the silver surface (Fig. S1).


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AgHX nanoparticles do not display bands corresponding to organic molecules. Only a strong band at 245 cm-1 due to the Ag-Cl- stretching appeared. Finally, also AgNS spectra are dominated by citrate bands but their intensity is much lower.

SERS spectra of HA Elliott Soil Humic Acid (SHA) SERS spectra of SHA on AgCT nanoparticles are characterized by two different spectral profiles, the first is observed at pH ≥ 5.0 and the second at pH < 5.0 (Figure 2). Three main bands at 1612, 1305 and 330 cm-1 are observed in the spectra obtained at pH over 5.0 (Fig. 2B). The strong and narrow bands at 1610 and 1305 cm-1 can be assigned to G and D bands, respectively, in agreement with Castiglioni et al33,34. These bands actually correspond to the polycyclic aromatic hydrocarbon (PAH) residues existing in the HA structure, as observed previously for other kinds of humic substance35,36,37. These bands are due to characteristic collective vibrational bands in graphite clusters of PAHs, giving rise to strong polarizability changes. In addition to the 1305 cm-1 band, other D bands are seen at 1250 and 1230 cm-1, attributed to the existence of different PAH structures33. Furthermore, the band at 330 cm-1 can be assigned to the skeleton motion of the polycyclic aromatic rings. The increase of the pH value in the SHA solution induces the carboxylic group ionization and H-bond disruption.24,26 As a consequence, a considerable structural rearrangement of HA macromolecules takes place. This entails the formation of uncoiling structures. Since PAH residues are mainly localized in the inner part of the macromolecular SHA structure24, they can be easily detected only after the opening of the SHA structure, which permits the access of PAHs to the Ag surface and a high absorption (see top diagram in Fig. 3). The SERS spectrum profile changes markedly at pH < 4.0. At these conditions, only two broad bands were observed at about 1610 and 1380 cm-1 (Fig. 2A), which are attributed to C=C stretching in aromatic rings and carboxylate groups interacting with the silver surface of the AgCT NPs. Thus, the bands seen at acidic pH were attributed to external polar groups (e.g., carboxyl, carbonyl, phenolic) existing at the periphery of HA matrices. These groups are able to interact with the citrate ions existing on the metal surface. The changes observed with the pH in the SERS of standard SHA studied here are significantly different than those observed for soild humic acids extracted by means of NaOH38, and those seen in the case of humic-like humic acids studied previously by us36. This indicates that the


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structure of the SHA resulting from the extraction suggested by the IHSS (basically extraction by using HF) leads to a significant structural change in the SHA. This is why we have performed an accurate pH study in order to get more information about this effect. The plot of the intensity of the G band (IG), normalized to the water band (IW) at 3400 cm-1 as function of pH, showed a significant increase toward alkaline pH values, reaching a maximum at ca 8.0 for AgCT (Fig. 3b). Above the latter pH value, a decrease in intensity was observed. This is attributed to the negative effect of OH- ions on the adsorption of SHA or the increasing negative charges in SHA upon ionization of phenol groups at pH above the latter one. A deeper analysis of the D band reveals that it is associated with ring breathing C-C stretching motions, according to the assignation of the PAH vibrations of large extent aromatic hydrocarbons33,34. Therefore, the presence of different D bands in SHA, at 1305, 1285 (shoulder), 1250 and 1230 cm-1, is attributable to the existence of PAHs bearing a different aromatic extent and different substituents. The latter bands were hardly observed in other HA, probably because of the higher amount of PAH structures in the SHA studied here. The intensity ratio between the D and G bands (ID/IG) also undergoes an interesting variation with pH (Fig. 3a). At alkaline experimental conditions, particularly in the 9.0-10.0 pH range, the D band increases, indicating that a further structural modification of SHA is taking place with pH. In fact, this effect can be associated with the release of large PAH moieties that are localized in more protected inner parts of the SHA macrostructure. These moieties are detected only after the ionization of phenolic OH group occurring above pH 9.039. In particular, the D band that undergoes the highest intensification on raising the pH is that at 1305 cm-1, which was related to the PAH structure with a larger extent33. The SERS spectra of SHA underwent a remarkable change at pH < 5.0. This was evident at pH 4.0 where the IG/IW ratio reached a minimum value, due to the SHA coiling and its subsequent aggregation (Fig. 3c). However, the large aggregation occurring at more acidic pH values (see top scheme in Fig. 3), due to the neutralization of the carboxylate groups to carboxylic groups, led to a large adsorption of SHA aggregates that again increased the SERS signal of the 1610 and 1380 cm -1 bands. However, this increase occurs until pH=2.0-3.0, and then, for lower pH’s a new decrease in the SERS intensity was observed. This is something that we coul not observe in other HA. The intensity of SERS bands is much weaker on AgHX nanoparticles (Fig. 3d). This is due to the high negative charge of AgHX, where chloride ions are adsorbed onto the surface (Fig. 1, top panel). The capability of SHA to be adsorbed on the surface is thus reduced because of the electrical repulsion. Although AgCT NPs are also negatively charged, CT is able to promote the adsorption of phenolic and benzoic moieties by H-bonds.


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Using AgHX nanoparticles, the PAH bands of humic substances are visible only at alkaline pH (Fig. S2 in Supporting Information). This is attributed to the large increase of negative charge induced by the ionization of acidic groups (carboxylate and phenols) and to the subsequent electric repulsion between these groups and the negative surface. Therefore, the pH 5 is a critical value favoring the transition in many aspects of HA: adsorption on surfaces and coiling processes. This result agrees well with the fact that this pH value corresponds also to the pK values of the soil humic substances reported elsewhere39.

Leonardate Humic acid (LHA) SERS spectra of LHA on AgCT showed very weak PAH bands at alkaline pH, while they were clearly dominated by citrate bands at acidic pH (Fig. S3 in Supporting Information). This result clearly points out that the interaction of LHA with the silver surface is less strong than in the case of SHA. Despite this weakness, a strong band is observed at 1590 cm-1 at acidic pH. The plot of the intensity of the latter band (IG), normalized to that of the water, reached a maximum at pH 2.0-3.0 on AgCT (Fig. 4A) as in the case of SHA. Using AgHX, the SERS spectra of LHA were very weak at pH below 5.0 (Fig. S4), conversely to the case of citrate capped NPs (Fig. S2). On AgHX, the maximum intensity of the 1590 cm-1 band occurred at pH 5.0 (Fig. 4A). Therefore, from these results it may be inferred that the LHA structure is more stable than that of SHA, being highly refractory to pH changes. The absence of PAH bands at high pH is also related to the structural resistance of LHA to the matrix opening, even if polycyclic aromatic groups may exist in the inner part of the LHA matrix, as deduced from the observation of weak bands at alkaline pH. The adsorption of LHA was higher on AgCT nanoparticles and it is only effective at low pH. This behavior can be attributed to the capping citrate ions existing on the silver surface, which may induce the formation of H-bonds between citrate ions and the external polar groups of LHA, while this is not possible on AgHX. In fact, the LHA behavior on the AgHX is that of a typical acidic compound. The adsorption increases at low pH until a pH close to the general pKa of the matrix (around 5.0). The dramatic decrease of intensity below this pH is attributed to the macromolecular nature of LHA. The large number of H-bonds between protonated carboxylic and phenolic groups occurring at very low pH leads to a massive aggregation of LHA which prevents them from the adsorption on the surface.


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SEF spectra The fluorescence intensity of the standard SHA analyzed here was lower than that measured in previous studies for humic-like substances or humic substances extracted by pyrophosphate. The variation of the fluorescence intensity (normalized to the water Raman band, IF/IW) is shown in Fig. 3e for SHA. The fluorescence undergoes an increase as the pH is lowered, reaching a maximum at pH= 5.0. Thus, the fluorescence emission undergoes an opposite trend in comparison to the Raman one. Below the latter maximum, the fluorescence emission decreases again due to the aggregation and multilayer adsorption that places the fluorophore emitting groups farther from the metal surface. This trend is also different that that observed for humic-like humic acids36.

Comparative effect of the substrate on the adsorption of HA The variation of the SERS intensity of SHA against the adsorbate concentration was followed on different nanoparticles in order to identify the influence of the interface chemical properties on the adsorption dynamic of these substances. Figure 4B shows the corresponding isotherms obtained for the different surfaces at different SHA concentrations. The sensitivity at a certain concentration of SHA decreases in the following order: AgCT > AgHX > AgNS, with AgCT being the substrate where the SERS intensity is highest. This result further underlines that the adsorption of SHA is higher on the citrate AgNPs. Furthermore, the SERS detection of SHA on AgCT was very sensitive, since the analysis reached a limit of detection close to 1 ppb. The high affinity of HA for AgCT is unusual, as the Ag NPs exhibit a negative zeta potential40,41 produced by the large amount of CT and its oxidation products adsorbed on the interface, which may avoid to a large extent the adsorption of the negatively charged humic substances. This is what also occurs on AgHX, where a large amount of adsorbed chloride ions decorates the surface (Fig. 1A). Therefore, the high affinity detected on citrate-covered nanoparticles is attributed to the establishment of a large amount of H-bonds with the oxygenated groups (carboxylic and phenolic groups mainly) existing in SHA (Fig. 5A). These bonds act as driving forces inducing the adsorption on the metal. Moreover, H-bonds seem to predominate over the electric repulsions induced by the negative charges existing in these molecules. These H-bond interactions are not possible in chloride capped AgHX NPs, thus accounting for the lower affinity on the latter surface. In the case of AgNS the adsorption of HA decreased due to the lower amount of CT adsorbed on the silver surface, as indicated by the low SERS signal of CT on this substrate (Fig. S1).


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Regarding the stability of the resulting suspensions, all colloidal suspensions were very stable in the presence of SHA or LHA. However, the suspension became more unstable upon the addition of nitrate, but the addition of this anion was necessary in order to activate the nanoparticles and render a high spectroscopic signal. The stability of the systems was very low when the humic acid was above 0.5 ppm, as deduced from the higher standard deviation found above the latter value (Fig. 4B), but at concentrations below the standard deviation, and then, the reproducibility, was very high.

Functionalization of silver nanoparticles with aliphatic diamines (AD)

Effect of AD on the SERS spectra of HA In order to further increase the affinity of Ag surfaces for humic substances, we tried to change the chemical properties of these surfaces by reversing the electric charge on their surface. For this purpose, aliphatic diamines (AD) of different length were employed (Fig. 1A). This experiment was carried out using AgHX nanoparticles in order to facilitate the AD adsorption on the surface. This is due to the presence of chloride ions on the surface which can induce an increase of the affinity for the silver surface through the formation of the corresponding ionic pair interactions (Fig. 5C). The SERS and SEF emissions were measured at different concentrations of SHA and LHA on the diamine-functionalized Ag NPs. These experiments were performed at pH 8.0 and using an AD concentration of 5x10-6 M, since at this concentration the maximum intensity was observed among all those employed. Figure 6A shows the SERS spectra and the intensity change of the G band (at 1615 cm -1) for SHA on AgHX functionalized with AD8, AD10 and AD12 is shown in Fig. 6B. The smallest diamines (AD6 and AD2) gave much lower intensities and their values were not inserted in the figure. The presence of diamines induced important changes in the SERS of the humic acids both regarding the intensity and the spectral profile. The Raman signal underwent a large enhancement on the diamine-functionalized surface, maximum for AD8 (Fig. 6A and 6B). This intensification is due to the affinity increase of HA to approach the interface, since the formation of different bonds between HA and the Ag surface take place (Fig. 5C): (a) electrostatic interactions between the positive interfacial surface of the AD-functionalized metal and the negative charges of ionized HA;


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(b) H-bonds between carboxylate and phenolic groups of HA and NH2 group in AD; (c) hydrophobic interactions between the AD alkylic chains and alkylic moieties in HA. The effect of the AD8 presence on the trend of the IG/IW ratio vs the SHA concentration is shown in Fig. 4B. AD8 induced an intensity intensification of 5 fold higher in comparison with what can be obtained using AgCT (the best performing NPs, among all the Ag substrates) at a SHA concentration of 3 ppm. Another important effect of the functionalization with diamines is the significant change on the SERS spectral profile (Fig. 6A). Although a broad and intense G band at 1615 cm-1, due to PAH residues, was still seen, new bands at 1365, 1239 and 1129 cm-1 were also observed. These bands are attributed to aromatic compounds rich in -OH, -COOH and –OCH3 functional groups. The appearance of these bands is due to the approach to the surface of these groups forming H-bonds with the amino groups exposed to the bulk, as depicted in Fig. 5C. Using AD10 and AD12, some of these bands were weakened, while others, i.e. at 1580 and 1380 cm-1, were enhanced. These changes and the lower intensification of the SERS intensity can be accounted for by taking into account the different polarity of these diamines and by assuming the higher hydrophobicity of these two diamines. In fact, these compounds may form highly ordered and more compact layers on the interface42, that may prevent the adsorption of the polar humic substances. In contrast, the higher polarity of AD8 leads to more appropriate layers to host the phenolic and carboxylate groups of SHA. On the other hand, AD2 and AD6 form layers with a possible lying down interaction, which is not so favorable for the SHA adsorption43. Fig. 7 shows the effect of the AD functionalization on the SERS intensity of LHA adsorbed on AgHX. In this case, a large enhancement of the SERS signal is also observed (see variation of the relative IG/IW versus SHA concentration in Fig. 7B). However, in contrast to the results observed for SHA, no significant variation of the spectral profile can be seen (Fig. 7A). This agrees with the results obtained on the effect of pH, indicating that the LHA structure is very stable and unlikely to become uncoiled. As in the case of SHA, a more sensitive effect was observed for AD8 in comparison with the more hydrophobic AD10 and AD12 diamines.

Effect on the Fluorescence emission of HA

The weak fluorescence emission of both standard SHA and LHA was highly enhanced when the adsorption of these substances increased upon functionalization of the silver surface by bifunctional diamines. This effect can be attributed to the formation of a diamine layer on the surface that acts as a good spacer to separate the HA from the metal surface and to avoid the energy


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transfer that quenches the fluorescence emission (See inserted scheme in Fig. 8B). Fig. 8 shows the fluorescence emission measured at 595 nm (excitation at 532 nm) for different diamines on SHA (Fig. 8A) and LHA (Fig. 8B). Although one must consider that a fluorescence intensification occurs on raising the concentration, due to a subsequent aggregation and agglomeration of HA (which also acts as an actual self-spacer of fluorophore groups), the positive charge of the surface provided by the diamine adsorbed on it also has a clear and significant effect. For instance, at a concentration of 1 ppm, the fluorescence is multiplied by a factor 50 for both SHA and LHA. As in the case of the SERS signal, AD8 is the diamine producing the greatest enhancement in the fluorescence. Furthermore, this effect is much higher for SHA than for LHA.

Concluding remarks The adsorption of HA on AgNPs depends on the nature of the chemical species existing in the surface of nanoparticles. This accounts for the different affinity of HA on AgNPs prepared by different methods. The affinity of Soil Humic Acids (SHA) is higher for citrate-capped AgNPs obtained after the reduction with trisodium citrate. This is due to the large amount of H-bonds established between carboxylic and phenolic –OH groups of SHA and the adsorbed CT. The effect of the surface functionalization, combined with the field intensification induced by LSPR, is crucial to ensure a large Raman enhancement. This is why the theoretically better morphology of star-shaped NPs does not provide SERS intensifications as high as in the case of spherical citrate-capped NPs. The modification of pH leads to important changes in the HA coiling and the aggregation, which dramatically modifies the affinity and the interaction mechanism of HA with the AgNP. SHA exhibits different SERS spectra on changing the pH due to the ionization of Phe-OH and HACOOH groups, which determine the uncoiling and the aggregation of the macromolecule as the pH is lowered. Conversely, LHA did not change the Raman profile, thus indicating a higher tightness of their structure. The behavior of standard SHA and LHA on metal NPs exhibits remarkable differences compared to HA extracted by different methods due to a structural rearrangement of the macromolecule which depends on this extraction procedure. The fluorescence follows an opposite evolution as compared to the Raman due to the fact that the fluorescence emission is higher in coiled HA structures. However, it decreases again at pH lower than 4.0 due to the intense aggregation of HA.


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The functionalization of AgNPs with alkyl diamines induces a great enhancement of both the Raman and fluorescence emissions for both SHA and LHA samples, due to the stimulated adsorption of HAs on the surface of AgNPs. Furthermore, a change in the interaction mechanism of SHA is observed, which can be attributed to the increase of H-bond interactions with the amino groups of AD and the higher amount of oxygenated groups (COOH and OH groups) in SHA as compared to LHA. Therefore, SERS and SEF techniques were both shown to provide important dynamical information to study the adsorption and structure modification of complex organic molecules on metal nanoparticles, with promising applications in many fields.

Acknowledgements This work was supported by the Spanish Ministerio de Economía, Industria y Competitividad (projects FIS2014-52212-R and FIS2017-84318-R). Supporting Information Control SERS spectra of the Nanoparticles of Ag in suspension; SERS spectra of SHA on AgHX nanoparticles at different pH; leonardate LHA on AgCT and AgHX nanoparticles at different pH. References 1

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R


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Figure 5. Adsorption schemes deduced for SHA interacting with AgCT at acidic pH (A), alkaline (B) and AD-functionalized AgHX (C)


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Figure 6. (A) SERS spectra of SHA (3 ppm) on AD-functionalized AgHX nanoparticles, [AD] = 5x10-6 M; (B) Variation of IG/IW vs. concentration in SERS spectra.


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Figure 7. (A) SERS spectra of LHA (5 ppm) on AD-functionalized AgHX nanoparticles, [AD] = 5x10-5 M; (B) Variation of IG/IW vs. concentration in SERS spectra.


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Figure 8. Variation of fluorescence emission (IF/IW) at 2000 cm-1 (595 nm) vs. concentration in SERS/SEF spectra of SHA (A) and LHA (B) on AD-functionalized AgHX nanoparticles. Excitation at 532 nm.


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Graphical Abstract G band D band

1500

1000

Wavenumber / cm


500 -1

Stimulated Adsorption of Humic Acids on Capped Plasmonic Ag

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