Bio-sensing of metal ions by a novel 3D-printable smartphone spectrometer

Highlights

• Detecting enzyme activity and performing kinetic studies is possible using a smartphone.

• Heavy metal enzyme inhibition cannot be described by common inhibition models.

• Smartphones enable the investigation of inhibition in educational labs.

• The 3D-printed smartphone photometer is capable of measuring reaction rates.

Abstract

The first lens-free smartphone spectrometer capable of observation and analysis of reaction kinetics and point measurements without additional electronics is demonstrated in this paper. The reaction rate is calculated and plotted in adjustable intervals by a smartphone app. The only additional materials needed for the spectrometer are produced by 3D-printing. The flashlight LED and the ambient light sensor of the smartphone are used for the absorbance measurements. The smartphone spectrometer is capable of measuring reaction rates by monitoring changes in the absorbance at wavelengths of approximately 425–560 and 625–675 nm. The restriction on specific wavelength is caused by the nonhomogeneous emission spectra of the smartphone’s flashlights LED. As a demonstration, the spectrometer and the individually programmed analysis software are used for reading and processing the signal of a urease bioassay developed for the detection and measurement of enzyme inhibitors. The inhibition caused by heavy metal ions is assessed by the smartphone through measurement of urease activity to a minimum activity of approximately 1 U ml⁻¹. For a detailed investigation of cumulative inhibition effects by low-cost analytical smartphone equipment, the results of mixed inhibition by heavy metals is compared to Bliss independence and Loewe additivity model theories. The study shows that cumulative inhibition effects of multi-component heavy metal solutions and different surface and supply water samples do not follow the conventional models of mixed inhibition effects. However, toxicity of mixed and single heavy metal ions in multi-component solutions, surface water samples, and supply water samples can be determined by the urease bioassay and the smartphone spectrometer. General water quality can also be rated using the same bioassay and equipment.

Introduction

The water crisis in Flint, Michigan, concerning lead pollution of the water supply is only a recent example of the impact heavy metal ions can have on the human body. Metallurgical pollution in soil and water can disrupt the ecological balance and have a negative impact on biodiversity and essential cycles of matter [1], [2]. Private households are sources of heavy metal emissions in addition to agricultural land and industrial manufacturing sites [3], [4]. Understanding the impact of heavy metal ions and raising public awareness are prerequisites to actively decreasing the contribution of households to overall emissions. Therefore, a simple and efficient way to determine their presence and quantify the mixed toxicity effect in water samples is crucial.

Today’s smartphones make a broad variety of sensors readily available. An everyday usage is the measurement of physical parameters, like sound or acceleration. The detection of biological parameters is less common and mostly relies on pattern recognition of camera recordings [5], [6]. For example, different microscopy applications [7], [8]. and antibody based systems such as LFIA [9], ELISA [10], antibody based Mie scattering [11], and fluorescent measurements [12], [13], [14] have been demonstrated. Most established concepts of “smartphones as spectrometers” require external equipment (i.e. a separate light source, optical components like lens or gratings and additional power supply [14], [15], [16], [17]).

In this paper, we outline the possibility of monitoring biological reactions in the form of bioassays or biosensors by relying on the internal light source and the ambient light sensor, thus eliminating the need for expensive optical components or an additional power supply. Cost effectiveness is achieved by drawing on an adjustable casing mainly consisting of 3D-printable components in addition to a simple filter foil to improve the signal-noise-ratio when detecting the diffuse flashlight LED spectrum (450–650 nm) via ambient light sensor. The adjustable casing allows for a compact structure, so combined with the low weight of the contraption, the high mobility of smartphones is preserved.

An enzymatic bioassay provides insights into the spectrometer’s detection capabilities. To this end, a urease based colorimetric enzyme assay is used to measure the influence of different heavy meatal ions at micro molar concentrations. Additional experiments were conducted to outline the interdependences of different heavy metal ions in the same sample when estimating the cumulative toxic effect.

In addition, while research facilities can draw on specialized equipment to conduct analyses, the educational system is tasked to include environmental, chemical, and biological analyses in educational settings for their students while facing financial restrictions. Consequently, financially feasible equipment needs to be developed to let these aims become achievable within the general education system. To allow for a broad application of the concept outlined within this paper, we limited our approach to require only low cost, 3D-printable components in additions to sensors already present in common smartphones. Consequently, the developed smartphone spectrometer introduces an innovative and cost-effective possibility to enrich the syllabus of different school subjects with a range of new experiments, while repurposing an everyday object for an educational use at the same time.