Natural polysaccharides and microfluidics: A win–win combination towards the green and continuous production of long-term stable silver nanoparticles

Highlights

• Flow system yields in better Ag nanoparticle size distribution and good stability.

• The synthesis of Ag NPs in flow requires a reaction time lower than the batch method.

• A green protocol together with a continuous set up lead to a long-term stable Ag NPs.

• Highly stable Ag Np suspensions were obtained for potential bactericidal applications.

Abstract

The present work combines the use of microfluidic reactors and green chemicals such as glucose and starch to achieve a continuous production of silver nanoparticles that have been successfully tested as excellent antibacterial agents against Escherichia coli. Those silver nanoparticles synthesized under continuous flow remained stable in terms of optical response, morphology and size distribution even after 48 months of storage at 24 °C without light protection. The best results in terms of colloidal stability were obtained after synthesizing those nanoparticles at 70 ºC in the presence of an excess of glucose when the continuous flow configuration was used to complete the reaction within 9 min. In contrast, analogous experiments carried out in batch conditions required a much longer period to achieve similar results but with much lower stability and a higher polydispersity, especially in the long-term storage range.

Introduction

Metal nanoparticles (i.e., mainly Au, Cu and Ag), have been extensively evaluated as antimicrobial agents. Infections caused by bacteria demand prolonged and not always successful treatments that affect negatively mortality and morbidity rates [1,2]. In particular, the bactericidal activity of silver nanoparticles (Ag NPs) has been attributed to the combined action of several mechanisms such as the production of reactive oxygen species (ROS), induced gene modification, cell-wall penetration and damage, and protein and DNA interactions [[3], [4], [5], [6]]. Great efforts are being devoted to synthesize Ag NPs as silver-ion reservoirs in paints, textile fibers and surgery materials in order to generate aseptic surfaces [7,8]. In addition, Ag NPs uses are extended to many others applications such as water treatment, coatings on catheters, prosthesis, on surfaces used in the food industry, food packaging, etc [9].

The wet chemical reduction of silver precursors in batch systems is the most widespread method for Ag NPs synthesis. The use of hazardous reducing agents, such as sodium borohydride (NaBH₄) or hydrazine (N₂H₄) is much extended. Furthermore, there is a strong motivation in applying the Green Chemistry concepts to in the material synthesis routes [10].

Consequently, different biological molecules have been evaluated as reducer agents for Ag NPs synthesis following safe, sustainable and eco-friendly procedures. For instance, microorganisms (bacteria, fungi actinomycetes, yeast and viruses,) acting as bioreactors have been reported to produce a variety of nanoparticles either intra or extra-cellularly [11]. Plant extracts from stems, flowers, leaves and seeds have also been studied as green reducers and stabilizers materials for NPs synthesis [12]. Although both synthesis employing either microorganisms or vegetable extracts are in agreement with the main green principles, the use of the former implies highly strict pH and temperature working ranges or lack of sufficient control on the chemical composition of the latter.

Polysaccharides have also been proposed as green reducer agents as they are able to replace the need for toxic solvents and they can also act as easy particle separation agent [13]. In the literature, there are a great number of examples where glucose and starch have been tested either as reducing agents [[14], [15], [16]], or as capping ligands [16,17]. Likewise, the combination of glucose acting as reductant and starch as co-stabilizer has been also evaluated [[18], [19], [20], [21]].

Glucose and starch have high market availability with stable formulation and low cost. In contrast, natural plant extracts that exhibit an analogous and promising role as effective reductors for the generation of NPs, require tedious, time-consuming purification steps that so far are preventing their commercialization.

The use of microfluidic systems for processing liquid phases to render nano-scaled materials has been taken into account as green process. Continuous material synthesis technology leads to a larger number of advantages compared with the batch-to-batch approach. Sebastian et al. [22] pointed out, “Microreactors constitute perhaps the enabling technology of the highest potential for liquid phase synthesis of Engineered nanomaterials (ENMs)” [23,24].

An intensive search of the reported works during the last ten years reveals the increasing efforts to find better alternatives for the Ag NPs production (Table S1). Although several authors studied glucose as reducing agent, the number of studies evaluating the use of glucose in a continuous synthesis is still scarce [18,19,21,[25], [26], [27], [28], [29]]. Horikoshi et al. [29] evaluated the Ag NPs synthesis employing glucose and microreactors and obtained narrow size distributions compared with the materials retrieved from conventional batch methods. Nevertheless, their studies did not validate the stability of the resulting Ag NPs suspensions for extended aging periods beyond 2–3 months [18,25,28].

Herein, we have combined the use of green chemicals such as glucose and starch and microfluidic reactors to achieve a continuous production of Ag NPs. The reaction condition effects on the Ag NPs optical response, morphology and size distribution were studied. The characteristics of the obtained suspensions were monitored in some cases for up to 48 months in order to evaluate their colloidal stability. A systematic comparison between the batch and continuous systems was performed. The Ag NPs synthesized were additionally tested as antibacterial agents against Escherichia coli. Therefore, a bacterial model (E. coli) irrespectively of its nature have just used in order to demonstrate the antimicrobial action of those materials. Actually, it is more difficult to treat gram-negative bacteria in comparison to gram-positive bacteria due to the presence of a membrane around the cell wall of gram-negative bacteria, which increases the risk of toxicity to the host being this membrane absent in gram-positive bacteria. In addition, porin channels are present in gram-negative bacteria, which can prevent the entry of drugs and antibiotics. These channels can also expel out antibiotics making much more difficult to treat in comparison to gram-positive bacteria. Finally, gram-negative bacteria possess both exotoxins and endotoxins but in case of gram-positive bacteria only exotoxins are produced [1]. Therefore, we chose E. coli due to the challenge that it represents and it was possible to corroborate that the glucose and starch do not inhibit the bactericidal effect of the synthesized nanoparticulated material.