ATP-Mediated Transient Behavior of Stomatocyte Nanosystems

Abstract

In nature, dynamic processes are ubiquitous and often characterized by adaptive, transient behavior. Herein, we present the development of a transient bowl-shaped nanoreactor system, or stomatocyte, the properties of which are mediated by molecular interactions. In a stepwise fashion, we couple motility to a dynamic process, which is maintained by transient events; namely, binding and unbinding of adenosine triphosphate (ATP). The surface of the nanosystem is decorated with polylysine (PLL), and regulation is achieved by addition of ATP. The dynamic interaction between PLL and ATP leads to an increase in the hydrophobicity of the PLL-ATP complex and subsequently to a collapse of the polymer; this causes a narrowing of the opening of the stomatocytes. The presence of the apyrase, which hydrolyzes ATP, leads to a decrease of the ATP concentration, decomplexation of PLL, and reopening of the stomatocyte. The competition between ATP input and consumption gives rise to a transient state that is controlled by the out-of-equilibrium process.

Keywords: ATP; nanomotors; nanoreactors; out-of-equilibrium systems; stomatocytes.

Scheme 1

a) Schematic of the transient deactivation and activation of a stomatocyte nanosystem mediated by ATP. b) Chemical structure of mPEG₄₅‐b‐PS₂₁₀, N₃‐mPEG₄₅‐b‐PS₂₁₅, PLL₂₀₀, ATP, and AMP.

Figure 1

a) TEM and b) SEM images of PLL‐modified stomatocytes. The red arrows indicate the opening of the stomatocytes. Scale bars=200 nm. c) Zeta potential changes of the ATP‐complexed PLL‐stomatocytes as a function of time upon the addition of apyrase. The PLL‐stomatocytes were pretreated with ATP (62.5 μm, r=1). Experimental conditions: 0.5 mg mL⁻¹ PLL‐stomatocytes, MES buffer (5 mm, pH 6.5).

Figure 2

Adaptive stomatocyte nanoreactors. a) Scheme of the ATP‐mediated responsive nanoreactors. HRP was encapsulated inside the cavity of the stomatocyte, and DMB was utilized as a substrate to evaluate the enzymatic reaction. (b) TEM image of HRP loaded PLL‐stomatocytes. Scale bar=200 nm. c) Asymmetric flow field‐flow fractionation (AF4) fractograms and radius of gyration (R _g) of empty PLL‐stomatocytes and HRP loaded PLL‐stomatocytes.

Figure 3

a) UV absorbance at 492 nm of the oxidation of DMB as a function of time upon the addition of different concentrations of ATP. b) Addition of apyrase switches nanoreactors back to the “ON” state. Experimental conditions: 0.5 mg mL⁻¹ PLL‐stomatocytes, MES buffer (5 mm, pH 6.5), [HRP]=5 U mL⁻¹ and [DMB]=400 μm.

Figure 4

a) Schematic of ATP‐regulated PtNP‐loaded stomatocyte nanomotors. b) TEM image of PtNP‐loaded PLL‐stomatocytes. Scale bar=100 nm. c) Asymmetric flow field‐flow fractionation (AF4) fractograms and radius of gyration (R _g) of empty PLL‐stomatocytes and PtNP‐loaded PLL‐stomatocytes.

Figure 5

a) MSD and velocity of PtNP‐loaded stomatocytes (solid line) and PtNP‐loaded PLL‐stomatocytes (dotted line) in the presence of H₂O₂ at different concentrations. b) MSD and velocity of PtNP‐loaded PLL‐stomatocytes in the presence of ATP at different concentrations. The extracted MSDs were used as a qualitative indication of how active our motors are as a result of their transient binding or unbinding to ATP.

Figure 6

a) Schematic of the ATP‐mediated adaptive stomatocyte nanomotors. b) Velocity of the nanomotors as a function of time on the addition of ATP (62.5 μm) to a PtNP‐loaded PLL‐stomatocyte solution in the presence of different concentrations of apyrase. c) Three cycles of an adaptive nanomotor system upon the repeated addition of ATP (62.5 μm) showing the out‐of‐equilibrium movement of the system. The arrows in (b) and (c) indicate the addition of ATP.