The formation of polyethyleneimine–trimethoxymethylsilane organic–inorganic hybrid particles

doi:10.1016/j.colsurfa.2013.04.022

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Volume 431, 20 August 2013, Pages 42-50

https://doi.org/10.1016/j.colsurfa.2013.04.022 Get rights and content

Highlights

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Hybrid particles were made with polyethyleneimine (PEI) and trimethoxymethylsilane (TMOMS).
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The effects of PEI concentration and different anions were studied.
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PEI is present at the surface of the silica hybrid particles made with TMOMS.
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Multivalent anions produced larger particles than monovalent anions.
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PEI aggregates with multivalent anions followed by silica growth.

Abstract

Polyethyleneimine–silica (PEI–silica) organic–inorganic hybrid particles were formed in the presence of multivalent anions, using trimethoxymethylsilane as the silica source. Here we present new understanding on the difference in particle formation when multi- and monovalent anions were used as well as providing clear evidence that anions other than phosphate may be used to produce PEI–silica particles, unlike the phosphorylated peptides involved in silication in nature. We also show that PEI–silica particle size increases with ionic strength when monovalent anions were used, but that the diameter is not ionic strength dependent with the use of multivalent anions. Furthermore, our results suggest that PEI–silica particle production involves the formation of polyethyleneimine aggregates with multivalent anions due to electrostatic interactions, producing sites from which silica particles can grow. In addition, for the first time zeta potential measurement data show the surface charge of the PEI–silica particles, giving evidence of the presence of the polyethyleneimine at the surface of the particles, without the need of an additional surface modification step. The superior understanding of silica particle growth in order to control the parameters needed to refine and improve the production of silica particles made using less toxic reagents is of great significance in silica particle fabrication and processing. In addition, the potential application of the polyethyleneimine on the silica particle surface is of great benefit since hybrid organic–inorganic silica particles have several applications from carbon dioxide capture to drug delivery in many fields of science, engineering and medicine.

Graphical abstract

Introduction

Spherical silica particles are used in a wide range of applications including coatings, cosmetics, catalysis, separation materials, enzyme immobilisation and sensors [1], [2], [3], [4], and the current global demand for them is forecast to grow by more than 6% per year [5]. For optimum use it is important to characterise the particles, in particular paying attention to particle size and morphology since physical, electrical, and optical properties of the silica particles vary with size. In addition, it is advantageous if the particles can be fabricated using neutral conditions and more environmentally friendly reagents than commonly used methods which often involve harsh reaction conditions [6], [7], [8].

Polyethyleneimine (PEI) is a polyamine which is very much in use at present for a number of important applications involving particles. For instance, PEI functionalised silica is used for carbon dioxide capture from the air [9] as well as for separation applications [10], and PEI functionalised particles are also used for gene delivery [11]. Furthermore, there is great potential to produce improved functionalised silica particles for these applications as well as biomedical materials, sensors and composite materials [10], [12]. However, it is vital to know the parameters which have to be tuned to be able to control the formation of the silica with the desired properties, such as size, shape and surface charge.

Recent work [4], [6], [13], [14], [15], [16], [17], [18] has demonstrated that polybasic peptide mimics such as PEIs and polyallylamine hydrochloride are efficient at directing silica deposition into nanoparticles in a similar manner to silaffin peptides involved in biomineralisation [2], [6], [19]. This is due to the multiple amine functional groups of PEI, as is found in nature where the silaffin peptides are highly decorated with polyaminated hydrocarbon chains [19]. The method of silica formation involves hydrolysis and condensation polymerisation of silane precursors to form silica nanostructures with different morphologies including particles, fibres and star-like architectures amongst others [1], [3], [14], [17]. However, there is very little work [6] on the use of PEI for controlled silica particle synthesis, or indeed on the effect of different anions on particle growth.

Phosphate is thought to act as a bridge between amine groups to form polyamine aggregates [13], [21], [22], [23], [24], [25]. It is believed that the polyamines in solution associate with multivalent anions forming aggregates and that silicic acid derivatives are adsorbed on to the aggregate or are incorporated into the aggregate which then solidifies into silica [13]. It has been proposed that the silica particle formation occurs after the aggregation of polyamines associated with multivalent anions via electrostatic interactions [13], [20], [23], [24], [26], [27], [28], [29]. However, hydrogen bonding interactions between the hydrogen phosphate anion and PEI are also thought to be important [3], [13], [21], [23], [25] since hydrogen bonding may occur between the silanol groups of silicic acid and proton donors such as amines in polyethyleneimine and free hydroxyl groups in other additives [30]. However, there is no previous work published on the hypothesis that trimethoxymethylsilane (TMOMS) polymerises around PEI-multivalent anion aggregates to form PEI–silica particles. In addition, most studies to date have used tetraethoxysilane or tetramethoxysilane for the silicic acid precursor [3], [9], [13], [23], [31]. Other studies have used sodium silicate [6], [32], silicon catecholate which dissociates to silicic acid [33], [34] and glycol modified silanes [35], but in general these silica sources do not produce uniform sized spherical particles, which was our aim. The results presented here are for trimethoxymethylsilane which is substantially less toxic and harmful than TMOS [8].

This work presents the study of bioinspired silication of TMOMS using the polyamine PEI, to form PEI–silica hybrid particles. Zeta potential measurements were made to determine the surface charge of the PEI–silica particles. Furthermore, the effects of PEI concentration, sodium phosphate buffer conditioning prior to incubation and the presence of different anions during particle synthesis were examined in order to gain insight into the mechanism of PEI–silica particle formation. ATR-FTIR and EDS measurements were also used to characterise the PEI–silica hybrid particles formed in this one-pot method.

Section snippets

Materials

Hydrochloric acid, PEI (molecular weight 25 kDa and 750 kDa), sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium nitrate, sodium sulfate, trisodium citrate, sodium chloride and PIPES (Piperazine-N,N′-bis(2-ethanesulfonic acid) were purchased from Sigma–Aldrich. The alkoxysilane precursor was TMOMS and was also supplied by Sigma–Aldrich. Unless otherwise noted, all reagent-grade chemicals were used as received and deionised water (18.2 MΩ) was used in the preparation of all aqueous

Results and discussion

The biomimetic silication of TMOMS using PEI was studied in order to gain insights into the mechanism of PEI–silica hybrid particle formation and to fully characterise the PEI–silica hybrid particles.

Conclusions

A study of bioinspired silication was presented in which PEI was used to induce the condensation polymerisation of hydrolysed TMOMS to form PEI–silica hybrid particles. The effects of PEI concentration, sodium phosphate buffer conditioning prior to silication, ionic strength and the presence of different anions during the silication reaction were examined in order to gain insight into the role of different anions in silica particle formation. Scanning electron microscopy, dynamic light

Acknowledgements

The authors wish to acknowledge the University of Newcastle Electron Microscope X-ray Unit. FN is the recipient of a University of Newcastle fellowship. TM acknowledges the Faculty of Science and IT, University of Newcastle for summer scholarship funding.

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The formation of polyethyleneimine–trimethoxymethylsilane organic–inorganic hybrid particles

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Results and discussion

Conclusions

Acknowledgements

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