Gytaute
Sirgedaite
a,
Martynas
Talaikis
a,
Gediminas
Niaura
a and
Lina
Mikoliunaite
*ab
aDepartment of Organic Chemistry, Center for Physical Sciences and Technology (FTMC), Sauletekio av. 3, LT-10257, Vilnius, Lithuania
bDepartment of Physical Chemistry, Faculty of Chemistry and Geosciences, Vilnius University, Naugarduko st. 24, LT-03225, Vilnius, Lithuania. E-mail: lina.mikoliunaite@chf.vu.lt
First published on 22nd November 2022
Surface enhanced Raman spectroscopy (SERS) is a powerful tool for the investigation of low concentration analytes. To improve this method, magnetic nanoparticles were introduced to the system. Magnetic nanoparticles together with plasmonic silver or gold nanoparticles or shells not only enhance the SERS signal but also facilitate to focus the sample and direct it to the place of need. In this work, different chemicals were used for the modification of magnetite in order to improve the attachment of plasmonic nanoparticles on the surface of magnetic nanoparticles. In addition to this, the modification using PEI was chosen for a further decoration procedure with different ratios of AgNPs and AuNPs in various sequences. Composites having silver, gold, and magnetite nanoparticles were obtained and the best modification conditions for optimal SERS signal were evaluated by employing multiwavelength excitation. It was noticed that the order of the decoration makes a difference in the obtained signal enhancement. These composites can improve the adsorption and sensing of different analytes as both silver and gold surfaces are accessible to the solution.
These structures have several advantages in comparison to usual gold or silver nanoparticles: using a magnetic field, the particles could be directed to any required place; if the solution of analyte is extremely low in concentration, the magneto-plasmonic structures could be used in the solution to catch the analyte molecules (using silver or gold part), and then, by applying an external magnetic field, the nanoparticles could be concentrated to one spot.
However, one of the main challenges in composing these structures is to obtain a core/shell structure instead of separate plasmonic and magnetite nanoparticles stuck together accidentally. To overcome this challenge, a complex organic synthesis process could be used,29 additional materials could be supplemented in the synthesis of magnetite9,30 or some modifications of the already synthesized magnetic core can be made. These changes are usually made using polymers,3,22 the most popular of which is polyethyleneimine (PEI).6,31 Thus, hybrid antibacterial magneto-plasmonic nanoparticles were obtained by covering the Fe3O4 core with a uniform 2 nm thick PEI shell through a self-assembly under sonication and subsequent deposition of Au@Ag nanoparticles of 3–5 nm diameter.6 In addition to this, silanes10,16,24,26,32 or SiO24,18 could be used.
In this work, magnetic nanoparticles were coated using various chemical materials during synthesis or immediately after the synthesis. During synthesis, some monomers: ethylene glycol, aniline, pyrrole, and polymer: polyethyleneimine were used while after synthesis, polymers: polyethyleneimine, polyvinyl alcohol, polyethylene glycol, and silanes: (3-aminopropyl) triethoxysilane, N-(3-(trimethoxysilyl) propyl) ethylenediamine, 3-(mercapto propyl) trimethoxysilane, and tetramethoxysilane were used. All these samples were decorated using negatively charged silver or gold nanoparticles and their coverage was evaluated using XRD, UV-Vis, and Raman spectroscopy methods. For further investigation, magnetite covered with PEI after synthesis was chosen. The particles were decorated with different concentrations of silver and gold colloids in different sequences as well as a mix of them. The obtained multiwavelength SERS results showed that the order of modification is important in the adsorption of analyte and the intensity of the SERS signal.
Then, different concentrations of silver or gold were used, and the ratio between water and colloid was changed. For example, the table (Table 1) with silver nanoparticles is presented; however, the same is valid for gold. In all the text, subscript means the percentage of colloid used in the modification procedure.
Abbreviation | Volume of silver colloid, mL | Volume of water, mL |
---|---|---|
AgNPs100 | 5 | 0 |
AgNPs75 | 3.75 | 1.25 |
AgNPs50 | 2.5 | 2.5 |
AgNPs25 | 1.75 | 3.25 |
SERS spectra of magnetite modified using different molecules and decorated with silver nanoparticles, were obtained using SNOM (Alpha300R, WiTec, ULM, Germany), equipped with a 532 nm laser source, 600 lines per mm grating, 45 μW power, and a 50×/0.8 NA objective. The signal acquisition time was 100 s.
The multiwavelength Raman microscopy characterization of the nanoparticles was accomplished by using an inVia Raman spectrometer (Renishaw, Wotton-under Edge, UK) equipped with a thermoelectrically cooled (−70 °C) CCD camera. Spectra were excited using 442 nm (with 2400 lines per mm grating and laser power of 0.53 mW), 532 nm (1800 lines per mm; 0.22 mW), 633 nm (1800 lines per mm; 0.05 mW), and 785 nm (1200 lines per mm; 0.09 mW), and 830 nm (830 lines per mm; 0.17 mW) laser radiation sources that were focused on the sample using a 50×/0.75 NA objective lens. The acquisition time was 30 s, which was obtained by averaging spectra from 30 different locations of the same sample of 1 s integrations. SERS intensity was quantified by dividing SERS intensity by Raman intensity (ISERS/IRaman) of the dominant 1582 cm−1 mode from the reporter molecule 4-MBA. Raman spectra were measured in 4-MBA (0.1 M) ethanol solution, and SERS spectra were measured for nanoparticles adsorbed with 4-MBA from the 2 mM aqueous solution. The intensities were additionally scaled to resolve the differences in acquisition times and laser powers and then normalized to 1. We assume the ratio of probed molecules, NRaman/NSERS, to be wavelength independent.
The modification of magnetite nanoparticles was carried out during the synthesis or immediately after obtaining Fe3O4 crystals. For the modification during synthesis, three monomers, aniline (AN), pyrrole (PY), and ethylene glycol (EG), were chosen together with one polymer – polyethylene imine (PEI). After the synthesis, three polymers, polyethylene glycol (PEG), polyvinyl alcohol (PVA), and PEI, were used as well as four different silanes:
• (3-aminopropyl)triethoxysilane (APTES)
• N-(3-(trimethoxysilyl)propyl)ethylenediamine (TMSPA)
• 3-(mercapto propyl)trimethoxysilane (MPTMS)
• Tetramethoxysilane (TMOS).
These materials were based on the works of other scientists. Xuan et al.2 used aniline for magnetite modification. We took two monomers: aniline and pyrrole. In addition to this, ethylene glycol was chosen for comparison with its polymer – PEG. All the used polymers, PVA, PEG, and PEI, are hydrophilic. For modifications of magnetic nanoparticles, in the literature, PEI is usually used.9,30,31
Finally, another type of material used for modification was silanes.16,24,32 Four different chemicals were chosen, and the chemical formula of each is presented in ESI† Fig. S1. The main difference was functional groups. TMOS has all the same –CH3 groups, while MPTMS has –SH, APTES and TMSPA have –NH2; only TMSPA has an additional –NH– in the middle of the chain. –NH2 group is known to have an affinity to silver as well as for gold, while the thiol group should make mainly gold-sulphur bonds. TMOS was used to check if silanes have any influence on nanoparticle adsorption due to other reasons, such as electrostatic. All the tested modifications are listed in Table 2. To see how the modification procedure changes the charge of the particles, zeta potential was measured after each modification and presented in Table 2, last column. Additionally, zeta potentials of bare magnetite, silver, and gold nanoparticles are presented. As could be seen from the data, as-synthesized magnetite, silver, and gold nanoparticles have negative zeta potentials, −25.0, −31.0, −31.0 mV, respectively. This means that in solution, they repel each other and do not form a composite (as was noticed in Fig. 1). The addition of monomers (AN, PY, and EG) to the magnetite synthesis solution barely changes the potential, and it stays negative; thus, due to electrostatics, monomer-modified magnetite should also repel plasmonic nanoparticles. The same is with polymers PVA and PEG, used after the synthesis. Only PEI, as a positive charge holding polymer, changes the magnetite surface charge to positive. However, there is a difference if PEI is used during the synthesis or after. PEI added to the initial solution changes magnetite zeta potential by 34.5 mV (reaching +9.5 mV), while if added after the synthesis, the polymer makes a 58 mV change to the positive direction (reaching +33.0 mV). All silanes, except TMSPA, reduce zeta potential, but the final value is still negative, while after the addition of TMSPA on magnetite, zeta potential changes to positive +3 mV. To sum up all the obtained results, due to zeta potential change and electrostatic interaction, only PEI and TMSPA modified magnetite nanoparticles should attract negative plasmonic nanoparticles; however, the attraction could be induced by other reasons, thus UV-Vis and XRD measurements were conducted for all the modified nanoparticles decorated either with gold or silver.
Abbreviation of the sample | Full name of modification material | Abbreviation of modification material | Moment of modification | Zeta potential of nanoparticles, mV |
---|---|---|---|---|
Fe3O4 | — | — | — | −25.0 ± 0.3 |
AuNP | — | — | — | −31.0 ± 0.5 |
AgNP | — | — | — | −31.0 ± 3.4 |
Fe3O4@AN | Aniline | AN | During synthesis | −21.1 ± 0.3 |
Fe3O4@PY | Pyrrole | PY | During synthesis | −22.5 ± 0.3 |
Fe3O4@EG | Ethylene glycol | EG | During synthesis | −20.9 ± 0.2 |
Fe3O4@PEID | Polyethylene imine | PEI | During synthesis | +9.5 ± 0.4 |
Fe3O4@PEG | Polyethylene glycol | PEG | After synthesis | −20.2 ± 0.2 |
Fe3O4@PEIA | Polyethylene imine | PEI | After synthesis | +33.0 ± 0.3 |
Fe3O4@PVA | Polyvinyl alcohol | PVA | After synthesis | −18. 0 ± 0.3 |
Fe3O4@APTES | (3-Aminopropyl)triethoxysilane | APTES | After synthesis | −15.0 ± 0.3 |
Fe3O4@TMSPA | N-(3-Trimethoxysilyl)propyl)ethylenediamine | TMSPA | After synthesis | +3.0 ± 0.9 |
Fe3O4@MPTMS | 3-(Mercapto propyl)trimethoxysilane | MPTMS | After synthesis | −14.4 ± 0.8 |
Fe3O4@TMOS | Tetramethoxysilane | TMOS | After synthesis | −26.1 ± 0.2 |
The UV-Vis spectra of initial materials, magnetite, AgNPs, and AuNPs, measured separately are presented in Fig. 2A. As could be seen, for magnetite nanoparticles, a gradual increase in absorption to the shorter wavelengths is seen. Also, a high scattering effect is present as all spectra are shifted upwards. Silver and gold nanoparticles have intensive and symmetrical peaks at 390 nm and 527 nm, respectively. According to other scientists, this represents 13 and 40 nm size nanoparticles.36,37 The size of nanoparticles was also calculated from SEM images (Fig. 2B and C) using the ImageJ program. The determined size was 13 ± 1 nm and 41 ± 3 nm for AgNPs and AuNPs, respectively.
![]() | ||
Fig. 2 (A) UV-Vis absorption spectra of magnetite (black), silver (green) and gold (pink) nanoparticles. SEM images of (B) gold and (C) silver NPs. Scale bar of both images is the same – 500 nm. |
After the modification procedure with monomers, polymers, and silanes, all the samples were decorated using silver or gold nanoparticles. For all the samples, XRD, UV-Vis, and SERS spectra were recorded, and obtained results are presented in ESI† (Fig. S2 – XRD spectra, Fig. S3 (ESI†) – UV-Vis absorption spectra, Fig. S4 (ESI†) – SERS). The modification success was evaluated if XRD peaks of silver or gold and plasmonic absorption (UV-Vis measurements) are visible. The summarized results are presented in Table 3.
XRD | UV-Vis | SERS | |
---|---|---|---|
Fe3O4@Ag | − | − | − |
Fe3O4@Au | − | − | − |
Fe3O4@AN@Ag | − | − | − |
Fe3O4@AN@Au | − | − | + |
Fe3O4@PY@Ag | − | − | + |
Fe3O4@PY@Au | − | − | + |
Fe3O4@EG@Ag | − | − | Very weak |
Fe3O4@EG@Au | − | − | − |
Fe3O4@PEID@Ag | + | + | + |
Fe3O4@PEID@Au | + | + | + |
Fe3O4@PEG@Ag | − | − | + |
Fe3O4@PEG@Au | − | − | + |
Fe3O4@PEIA@Ag | + | + | + |
Fe3O4@PEIA@Au | + | + | + |
Fe3O4@PVA@Ag | − | − | − |
Fe3O4@PVA@Au | + | + | + |
Fe3O4@APTES@Ag | + | + | + |
Fe3O4@APTES@Au | + | + | + |
Fe3O4@TMSPA@Ag | + | + | + |
Fe3O4@TMSPA@Au | + | + | + |
Fe3O4@MPTMS@Ag | − | − | + |
Fe3O4@MPTMS@Au | + | + | + |
Fe3O4@TMOS@Ag | − | − | Very weak |
Fe3O4@TMOS@Au | − | − | − |
The information obtained from XRD agree the results measured with UV-Vis spectrometer. The plasmonic nanoparticles attached to the surface of magnetite when PEI was used for modification during or after the synthesis as well as silanes: APTES and TMSPA. However, MPTMS showed a signal only with AuNPs, as –SH has a higher affinity to gold, the signal was also registered using PVA and modification with gold NPs. Nonetheless, 4-MBA SERS spectrum was registered for more samples. In addition to PEI, APTES, and TMSPA modification, 4-MBA SERS spectrum was registered for magnetite samples modified with pyrrole and PEG decorated with both silver and gold NPs. Separately, gold decorated magnetite, modified with AN, provided a signal, while for AgNPs decoration, magnetite modified with EG (weak), TMOS (weak), and MPTMS showed the signal. Thus, although XRD and UV-Vis techniques are not able to register the signals, a small amount of nanoparticles possibly attaches to the surface of PY, PEG, TMOS (for AgNPs), AN (for AuNPs), EG (for AgNPs), and MPTMS (for AgNPs) modified magnetite and produces noticeable SERS signal.
For further investigations, only the sample modified with PEI after the synthesis was chosen, as it showed one of the highest SERS intensities. The unprocessed SERS data are presented in ESI† Fig. S5. The magneto-plasmonic nanoparticles, which have PEI added during the magnetite synthesis (Fe3O4@PEID), although showed satisfactory SERS intensity, were difficult to manage—they adhered plastic labware. It was difficult to control the number of NPs; thus, this particular nanoparticle type was eliminated from further investigation. As only the magnetite with PEI added after the synthesis will be analyzed further, we simplify the abbreviation to Fe3O4@PEI.
Firstly, different concentrations of plasmonic nanoparticles were evaluated. The modification procedure was conducted using pure colloid solution (100%) or diluted with ultrapure water to obtain 75%, 50%, or 25% solutions. The results with gold or silver nanoparticles are presented in Fig. 3. From the XRD data, the gradual increase of plasmonic nanoparticle peak for both samples matches the increasing nanoparticle concentration in the modification solution. This means that more nanoparticles adsorb to the surface of the magnetite each time. However, the intensity of the plasmonic nanoparticle peaks in comparison to magnetite differs. During the decoration procedure, the amount of magnetite maintained the same, thus all XRD spectra were normalized to the highest (35.4°) Fe3O4 peak. The intensity of AgNPs peaks was always lower in comparison to magnetite, while the AuNPs peak was always higher. This might be due to several reasons; firstly, the gold could adsorb more readily to the surface of magnetite in comparison to AgNPs; the initial concentration of gold colloid was higher, and finally, the size of gold nanoparticles was larger.
UV-Vis spectra do not show very obvious differences (Fig. 4). With silver NPs, the absorption curve form remains similar for all the samples, and only the intensity for Fe3O4@PEI@AgNPs25 is smaller. But this might be due to the lower concentration of the sample during UV-Vis measurements. The absorption band is composed of two peaks at 440 and 650 nm. The first one is related to the absorption of individual silver nanoparticles, while the second one is related to the plasmonic nanoparticles in close proximity to each other at the magnetite surface. This interaction was described in our previous and other research works.38,39 Meanwhile, the increase in the concentration of gold nanoparticles resulted in higher intensity of absorption for two visible plasmonic peaks at 550 nm and 720 nm. The explanation for the appearance of these two peaks is the same as for the AgNPs.
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Fig. 4 UV-Vis absorption spectra of PEI modified magnetite decorated using different concentration of (A) silver or (B) gold nanoparticles. |
The next part of this work was to decorate magnetite with two different sorts of nanoparticles. For further investigation, samples modified with 50% AgNPs or AuNPs solutions were chosen to leave an additional place for the other nanoparticles to adhere. Three different methodologies were tested for the decoration: (i) magnetite was capped with silver and then with gold NPs, (ii) magnetite was capped with gold and then with silver NPs and (iii) magnetite was capped with a mixture of both silver and gold NPs. We choose to vary the concentration of the second plasmonic NPs solution. All the prepared samples and numeration are presented in the ESI,† Table S1. For these samples, again XRD and UV-Vis measurements were conducted. In Fig. 5, the XRD spectra of twice decorated nanoparticles are presented. For both sorts of samples, the Au and/or Ag XRD peak dominates over the magnetite peak; however, it is impossible to qualitatively or quantitatively decide which type of plasmon nanoparticles dominate. Their XRD peak positions almost match each other; thus, this method is not suitable for the evaluation.
UV-Vis spectra (Fig. 6) of both types of modification showed a similar shape of curves with a weak peak at 400 nm (AgNPs), an intensive peak at 550 nm (AuNPs), and a shoulder at 720 nm (plasmonic interactions). However, two samples modified with the highest concentrations of AuNPs (Fe3O4@PEI@AgNPs50@AuNPs75 and Fe3O4@PEI@AgNPs50@AuNPs100) precipitated very fast, and even with ultrasound, it was impossible to separate nanoparticles. Thus, these samples showed very low intensity in absorption spectra and were eliminated from the following SERS measurements.
Fig. S6 (ESI†) compares Fe3O4@PEI@AgNPs50@AuNPs50, Fe3O4@PEI@AuNPs50@AgNPs50, and the nanoparticles where magnetite was capped with a mixture of Ag and Au NPs. These latter nanoparticles showed quite high XRD intensity of Au and/or Ag peak, and the UV-Vis spectra were similar to the other two samples.
![]() | ||
Fig. 7 Hysteresis loop of the magnetite (green), magnetite modified with PEI (red) and magnetite decorated with silver nanoparticles: Fe3O4@PEI@AgNPs50 (black). |
Due to the maintained magnetic properties of the sample, over time, they tend to stick together and precipitate. However, this process is not irreversible. If the bottle is heavily shaken or added to the ultrasonic bath for a few seconds, the nanoparticles are separated and could be further used without any alteration.
![]() | ||
Fig. 9 SERS spectra of 4-MBA adsorbed on Au and Ag nanoparticles and composite nanoparticles with varied Ag concentration. The bottom spectra are 1075 cm−1 mode intensity-normalized. |
To get a deeper understanding of nanoparticle properties, multiwavelength SERS measurements were carried out. Composite iron oxide, silver, and gold nanoparticles, regardless of their exact composition, exhibited the greatest surface enhancement towards near-infrared portion of the spectrum, with 785 nm excitation being the highest for most of the nanoparticle types. Unsurprisingly, at fixed AuNPs concentration but increasing AgNPs, incrementally higher enhancement was found practically at each laser excitation wavelength (Fig. 10A). However, that was not the case for Fe3O4@PEI@AgNPs50@AuNPsy, where AgNPs were added first and then the AuNPs. A successive increase in AuNPs concentration from y = 25 to 50 had a diminishing effect on SERS enhancement (Fig. 10B). One might suspect that the addition of a second plasmonic nanoparticle type might affect the quantity of nanoparticles that were originally adsorbed on Fe3O4@PEI; in this case, the deposition of the second layer of AuNPs would minimize AgNPs already present on Fe3O4@PEI cores. Since silver has a higher enhancement factor than gold,46,47 its substitution with AuNPs is expected to decrease the normalized SERS intensity. Additionally, the 4-MBA preference towards Ag in our setup has been demonstrated in Fig. 9.
To explain how the AuNPs and AgNPs capping order affects the SERS signal, we prepared and tested composite nanoparticles where (i) AuNPs were added to the Fe3O4@PEI first, and then the AgNPs, (ii) AgNPs were added first, then the AuNPs, and (iii) AuNPs and AgNPs were first mixed together and then added to the iron oxide nanoparticle solution (Fig. 11). Apparently, the composition that had first added AuNPs and then the AgNPs showed the highest SERS intensity, while the other two were similar. It should be noted that the direct connection of UV-Vis absorption maximum and enhancement of SERS signal at certain excitation wavelengths cannot be accomplished accurately because the spatial plasmon resonance distribution for these methods is different; surface-like for SERS and bulk-like for UV-Vis.48
SERS characterization of nanoparticles indicated that reporter molecule 4-MBA has a preference towards the Ag part of the composite nanoparticles. Such an observation, together with the higher Ag enhancement factor, comes into play when the decoration sequence for Fe3O4@PEI is considered. Indeed, we show that sequence matters, as the nanoparticles prepared by capping iron oxide with AuNPs and then AgNPs exhibited noticeably higher SERS intensity compared to the ones prepared in reverse order or the ones where AuNPs and AgNPs were added simultaneously. We demonstrate that incrementally higher AgNPs concentration in composite nanoparticles has a positive effect on the SERS intensity, especially with laser excitation source in the near-infrared.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nj04368h |
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2023 |