BioNP - Results

1. Formation by Laser Ablation in Liquid (LAL) and characterization of cysteine-gold nanoconjugates

One of the first objectives was to use LAL in order to form gold nanoparticles conjugated with cysteine (Cys-AuNPs). This system was chosen in order to develop and validate the method, due to the fact that gold NPs are largely studied and their interaction with the S-H containing molecule is well documented. The results obtained in this phase were used to understand the importance and influence of the ablation parameters and liquid phase composition on the properties of the produced bionanoconjugates and on the process efficiency.

Cys-Au nanoparticles have been formed under various conditions and have been characterized by UV-Vis absorption, Transmission Electron Microscopy, Dynamic Light Scattering, X-ray Photoelectron Spectroscopy.

The ablation of a metallic Au target immersed in a cysteine solution with concentration bellow 0.1 mM produces some “chain-like” structures, composed of incompletely separated nanoparticles. At low concentrations, the number of Cys molecules in the solution is not enough for a full coverage of nanoparticles that continue to grow and coalesce. The nanoparticles are well separated at concentrations higher than 0.1mM, with smaller particles being formed in more concentrated solutions.

(a) Optical absorption of AuNPs and Cys-AuNPs obtained at different Cys concentrations;
(b) Surface Plasmon Resonance (SPR) peak

One of the most important properties of AuNPs (defining a wide range of applications) is their Surface Plasmon Resonance (SPR). This gives a specific absorption spectrum, with a maximum between 520-530 nm, depending on the nanoparticles size, shape, surface chemistry, dielectric constant of the solvent etc. In our case, as predicted by theory, the smaller size of nanoparticles obtained in more concentrated solutions decreases by a few nanometers the wavelength of maximum absorption and the wavelength shift indicates the complete separation of nanoparticles.

The efficiency of nanoparticles formation by LAL can be influenced by adjusting the quantity of ablated metal. More material will be ablated with a higher energy laser, but this can also influence the quality of obtained nanoparticles. In order to study this effect, we produced Cys-AuNPs at three laser fluences: 1.7, 1.9 and 2.3 J/cm², while keeping all the other parameters unchanged. The value 1.7 J/cm² corresponds to the ablation threshold; at lower fluences, no nanoparticles are formed.

DLS measurements show that by increasing the fluence above the threshold, the nanoparticles formed are smaller and have better polydispersity. Very small differences were noticed between the nanoparticles formed at 1.9 and 2.3 J/cm². Similar observation can be made by analyzing the optical absorption spectra.

Cys-AuNPs formed at 1.9 and 2.3 J/cm² are qualitatively very similar; only a small quantitative difference is noticed, indicating that the increase of laser energy does not affect the quality, only the number of formed nanoparticles. We can therefore optimize the efficiency of Cys-AuNps formation by increasing the laser energy, without modifying significantly the formed nanoparticles.

Formation of magnetic nanoparticles by laser ablation of metallic iron immersed in water was proven by the observable color change of the liquid during irradiation. In the absence of a capping agent, the NPs rapidly agglomerate (in a few minutes). The magnetic properties are proven by particles migration in the presence of a magnetic field.

In order to stabilize the formed nanoaparticles and to avoid their rapid agglomeration, we used citric acid as capping agent. Hydrophilic citric acid-coated stable magnetic nanoparticles (MNP-CA) were obtained by laser ablation of metallic iron immersed in aqueous solution of citric acid. By this method, we obtained spherical particles, with one size distribution around 40-60 nm. The increased stability of the colloidal solutions in proven by the high value of the Zeta potential (-55mV).

The morphology of the nanoparticles can be is studied by TEM. It is evident that the particles have a spherical shape and they present a multicore structure. This can be explained by the high reactivity of laser ablated Fe, leading to the formation of an outer oxide layer. The electron diffraction (ED) pattern indicates the high crystallinity and crystal order of the sample.

The chemical composition of the MNP-CA as indicated by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) show the presence of elements of the precursors, respectively Fe, C, and O, in the form of iron oxides, FeOOH and COOH species. There was no indication of citric acid decomposition (possible because of the high temperature and pressure locally attained due to the Fe ablation).
The binding of the citric acid to the surface of the particles was proven by comparing the FTIR spectra of pure citric acid and of MNP-CA.

The shift of the C=O peak (from 1728 to 1618 cm^-1) is explained by the carboxylate ion formation (that renders a partial single bond character to the carbonylic group), indicating the binding of citric acid to the surface of the particles by chemical sorption of carboxylate ions.

Reference: Anamaria Durdureanu-Angheluta; Claudia Mihesan; Florica Doroftei; Andrei Dascalu; Laura Ursu; Michalis Velegrakis; Mariana Pinteala, Formation by Laser Ablation In Liquid (LAL) and Characterization of Citric Acid-Coated Iron oxide Nanoparticles, Rev. Roum. Chim., 2014, 59(2), 151-159
http://revroum.lew.ro/wp-content/uploads/2014/2/Art%2010.pdf

The stability and magnetic properties of the nanoparticles are visible in the following pictures. Nanoparticles formed in the absence of stabilizer agglomerate within minutes after formation (a). Nanoparticles produced in solution containing citric acid are stable as formed (b) and after washing the stabilizer in excess in solution (c). They are attracted by a magnet placed in their vicinity (d) and the magnetization curve of 0.5 ml solution is presented in figure (e).

2.a.3. Iron oxide nanoparticles conjugated with Polyethylene glycol (PEG-FeNP)

PEG-FeNP were obtained in two ways:

In both cases, the solution was moderately stable (the particles agglomerated within hours, slower than in the absence of PEG). TEM images of the structures obtained are presented bellow.

Nanoparticles obtained by laser irradiation of Fe in (a) PEG solution of 0.25 mg/ml; (b) PEG solution of 0.5 mg/ml; (c) in pure water followed by immediate adding of PEG to final concentration of 0.25 mg/ml.

From TEM images it is noticed that 0.25 mg/ml PEG is not enough to well stabilize and separate the NPs. By adding PEG to nanoparticles formed in pure water, we obtain some inhomogenous structures formed by embedding the Fe nanoparticles in the polymer matrix. Similar structures were obtained by direct ablation of Fe in a polimer solution of double concentration.

2.b. Chemical synthesis of magnetic nanoparticles
Hydrophobic iron oxide nanoparticles have also been synthetized using a chemical technique, which consisted in grounding the powder reagents (iron II, III salts) in the presence of the oleylamine-oleic acid adduct and NaOH. Following separation and purification of the nanoparticles, the hydrophobic shell was replaced by 3-aminopropyltriethoxysilan generating iron oxide hydrophilic nanoparticles. The Fe²⁺/Fe³⁺ molar ratio used in the synthesis process influenced the formed nanoparticles size, with smaller NPs being formed from a Fe²⁺ poorer mixture.

Reference: A. Durdureanu-Angheluta, A. Dascalu, A. Fifere, A. Coroaba, L. Pricop, H. Chiriac, V. Tura, M. Pinteala, B. C. Simionescu, Progress in the Synthesis and Characterization of Magnetite Nanoparticles with Amino Groups on the Surface, J. Magn. Magn.Mater. 324, 1679–1689 (2012)
http://dx.doi.org/10.1016/j.jmmm.2011.11.062

3. Study of laser-ablated iron ions interaction with oxygen in gas phase

These results were partially obtained during several research stages performed in the Clusters & Molecular Dynamics group (dr. Michalis Velegrakis) from Institute of Electronic Structure and Laser, Foundation of Research and Technology – Hellas, Heraklion (Greece).
We formed iron oxide clusters (Fe(O₂)_n⁺) by mixing the ablation plume (the gas phase formed following laser ablation of a pure iron target) with an O₂ beam produced by a nozzle. The distribution of the resulting clusters is studied by time of flight mass spectrometry (TOF-MS) and information about their stability and structure is obtained by collision induced dissociation (CID) experiments with a noble gas. In highly energetic conditions, each collision leads to the cluster’s fragmentation, therefore the measured fragmentation cross section is equal to the collision cross section; furthermore, the latter is a good approximation of the geometrical cross section and consequently is a valuable indication on cluster’s structure. Two techniques were used for the measurement of fragmentation cross section: (1) the collisions take place in a collision chamber and fragments are separated by a reflectron and (2) the cluster beam crosses perpendicularly a second beam of noble gas atoms and fragments are separated by a retarding energy analyzer. The first technique is well known and has been validated by many previous studies but has the main disadvantage of measuring only one cluster at a time, leading to long and expensive experimental runs. Moreover, the necessary mass separation of individual clusters leads to losses and does not allow the measurement of less abundant species. The second approach, implemented in our laboratory, has the advantage of measuring all clusters in one series of experiments.

(a) Mass spectrum of iron oxide clusters; (b) Comparison between the fragmentation cross sections of Fe(O₂)_n⁺ clusters measured by two experimental techniques and geometrical cross sections of structures obtained by ab-initio calculations

Reference: M. Velegrakis; C. Mihesan; M. Jadraque, Collision-Induced Dissociation Studies on Fe(O₂)_n⁺ (n=1-6) Clusters: Application of a New Technique Based on Crossed Molecular Beams, J. Phys. Chem. A, 117, 2891–2898 (2013)
http://dx.doi.org/10.1021/jp310617e

Similar results were obtained for two other series of clusters: NbO_y⁺ and YO_y⁺.

Comparison between the fragmentation cross sections and geometrical cross sections of structures obtained by ab-initio calculations for (a) NbO_n⁺ and (b) YO_n⁺.

Reference: Claudia Mihesan, Pavle Glodić, Michalis Velegrakis, Collision Induced Dissociation of Nb_xO_y⁺ (x=1,2, y=2-12) Clusters: Crossed Molecular Beams and Collision Cell Studies, accepted, Appl. Phys. A, July 2014

By confirming the values obtained experimentally with the geometrical cross sections obtained by calculations, we proved that our method can be successfully used to measure the fragmentation cross section of various metal oxide clusters.

4. Study of the ablation plume by optical diagnosis
A better understanding of Laser Ablation in Liquid (LAL) process can largely contribute to improving NPs formation. In the idea of future research on optical diagnosis of the laser ablation plume in liquid, we coupled Laser Induced Breakdown Spectroscopy (LIBS) analysis with Mass Spectrometry, for confirmation of the detected species. In the first phase, the coupling was applied to several commercially available pigments – systems that are well known and largely studied in the frame of art conservation efforts.

Reference: O. Kokkinaki; C. Mihesan; M. Velegrakis; D. Anglos, Comparative Study of Laser Induced Breakdown Spectroscopy and Mass Spectrometry for the Analysis of Cultural Heritage Materials, J. Mol. Struct., 1044, 160–166 (2013)
http://dx.doi.org/10.1016/j.molstruc.2013.01.069

The results presented in the above-mentioned paper were obtained during a research stage performed at Institute of Electronic Structure and Laser, Foundation of Research and Technology – Hellas, Heraklion (Greece), in collaboration with dr. Michalis Velegrakis and dr. Demetrios Anglos.