Living and Regenerative Material Encapsulating Self-Assembled Shewanella oneidensis-CdS Hybrids for Photocatalytic Biodegradation of Organic Dyes

Abstract

Reductive biodegradation by microorganisms has been widely explored for detoxifying recalcitrant contaminants; however, the biodegradation capacity of microbes is limited by the energy level of the released electrons. Here, we developed a method to self-assemble Shewanella oneidensis-CdS nanoparticle hybrids with significantly improved reductive biodegradation capacity and constructed a living material by encapsulating the hybrids in hydrogels. The material confines the nano-bacteria hybrids and protects them from environmental stress, thus improving their recyclability and long-term stability (degradation capacity unhindered after 4 weeks). The developed living materials exhibited efficient photocatalytic biodegradation of various organic dyes including azo and nitroso dyes. This study highlights the feasibility and benefits of constructing self-assembled nano-bacteria hybrids for bioremediation and sets the stage for the development of novel living materials from nano-bacteria hybrids.

Keywords: Shewanella oneidensis; biodegradation; hydrogel; living material; nano-bacteria hybrid.

Figure 1

Schematic showing the workflow to construct engineered living materials (ELMs) with encapsulated nano-bacteria hybrids. The CdS nanoparticles biomineralized by Shewanella oneidensis are deposited on the surface of cells to self-assemble nano-bacteria hybrids. The hybrids are encapsulated in an alginate hydrogel containing nutrients to form ELMs. Nano-bacteria hybrids in the ELMs can remove contaminants via photocatalytic reductive degradation.

Figure 2

Characterization of the CdS-Shewanella oneidensis hybrids self-assembled under pseudo anaerobic conditions: (a) TEM image of nano-bacteria hybrids showing self-biomineralized CdS nanocrystals deposited on the cell surface; (b) Electron diffraction pattern of the CdS nanocrystals in nano-bacteria hybrids from HRTEM results. The d-spacing of the CdS nanoparticles is 3.3 Å which aligns with the (111) plane of face-centered cubic CdS; (c) SAED analysis of the CdS nanocrystals in nano-bacteria hybrids. The indices are assigned to the diffraction rings in accordance with the face-centered cubic lattice of CdS.

Figure 3

Photocatalytic degradation of trypan blue by nano-bacteria hybrids: (a) Change in concentration of trypan blue after treatment with nano-bacteria hybrids and controls. Nano-bacteria hybrids were incubated in a mineral medium solution containing 20 mg/L trypan blue and irradiated by a tungsten filament lamp. The absorbance of the reaction mixture at 583 nm was used to determine the change in concentration of trypan blue. Control groups included trypan blue only (with light), Shewanella oneidensis only (with light), CdS only (with light), and nano-bacteria hybrids (without light). The photocatalytic degradation performance of nano-bacteria hybrids was substantially improved compared to that of wild-type S. oneidensis cells; (b) The concentration of trypan blue after 12 h and 24 h of treatment was compared with the initial concentration to assess trypan blue degradation efficiency. The concentration was determined by comparing it to a trypan blue calibration curve generated based on Abs_{583 nm}. The photocatalytic degradation efficiency of nano-bacteria hybrids was 33 times better than that of wild-type S. oneidensis cells. The data are presented as mean ± s.d (n = 2).

Figure 4

Photocatalytic degradation performance of engineered living materials (ELMs) with encapsulated nano-bacteria hybrids: (a) Change in concentration change of trypan blue after treatment with ELMs and controls in mineral medium solution. The ELMs were incubated in a mineral medium solution containing 20 mg/L trypan blue and irradiated by a tungsten filament lamp. Control groups included hydrogels (light), Shewanella oneidensis encapsulated in hydrogels (light), CdS encapsulated in hydrogels (light), and ELMs (no light). The photocatalytic degradation performance of ELMs was significantly improved compared to that of wild-type S. oneidensis cells encapsulated in hydrogels; (b) Change in concentration of trypan blue after treatment with ELMs and controls in water; (c) Change in concentration of trypan blue after treatment with ELMs and controls in PBS solution; (d) The concentration of trypan blue after 24 h of treatment was compared with the initial concentration to assess the trypan blue degradation efficiency. The concentration was determined by comparing it to a trypan blue calibration curve generated based on Abs_{583 nm}. The photocatalytic degradation efficiency of ELMs was not affected by the media (i.e., simulated wastewater, water, and PBS solution). The data are presented as mean ± s.d (n = 2).

Figure 5

Recycle and recharge engineered living materials (ELMs) after the photocatalytic degradation process: (a) Recycling and reuse cycle of ELMs. After degrading trypan blue for 24 h, the ELMs were collected and used to degrade fresh trypan blue solution. The photocatalytic degradation efficiency was lower than that of freshly prepared ELMs. The recycled ELMs were incubated in Luria Bertani (LB) medium solution. After a 24-h incubation, the recharged ELMs were used for the photocatalytic degradation of trypan blue; the observed efficiency was comparable to that of freshly prepared ELMs; (b) Degradation efficiency of freshly prepared ELMs, used ELMs, and recharged ELMs after 24 h of treatment. The photocatalytic degradation efficiency of ELMs decreased after incubation with trypan blue but recovered after incubation with LB medium solution. The data are presented as mean ± s.d (n = 2).

Figure 6

Cell viability in recharged engineered living materials (ELMs). After photocatalytic degradation of trypan blue, spheres were incubated in a culture medium solution. The recharged ELMs spheres were washed with PBS solution and dissolved using EDTA solution. The released cells were co-stained with SYTO 9 and PI and subsequently analyzed under a confocal laser scanning microscope. The majority of the hybrid cells in the recharged ELMs were alive.

Figure 7

Assessment of the photocatalytic degradation performance of engineered living materials (ELMs) stored under different conditions. The concentration of trypan blue after 24 h of treatment was compared with the initial concentration to assess the trypan blue degradation efficiency. The concentration was determined by comparing it to a trypan blue calibration curve generated based on Abs_{583 nm}. The ELMs could retain their photocatalytic degradation capacity for up to 28 days when stored at temperatures lower than 4 °C. The data are presented as mean ± s.d (n = 2).

Figure 8

Assessment of the regenerative properties of engineered living materials (ELMs): (a) Change in concentration of trypan blue after treatment with the 1st and 10th generation of regenerated ELMs. Live cells were extracted from recycled ELMs and used to regenerate bacterial culture for the biosynthesis of nano-bacteria hybrids. The as-prepared nano-bacteria hybrids were encapsulated in alginate hydrogels to regenerate ELMs. The regeneration process was repeated for 10 generations; (b) The concentration of trypan blue after 24 h of treatment was compared with the initial concentration to assess the trypan blue degradation efficiency. The concentration was determined by comparing it to a trypan blue calibration curve generated based on Abs_{583 nm}. Regenerated ELMs exhibited photocatalytic degradation efficiency comparable to that of freshly prepared ELMs. No significant difference was observed between the degradation performance of different generations of ELMs. The data are presented as mean ± s.d (n = 2).