Cannot pass string value from c# to JavaScript using JSON

I am developing a WindowsPhone App using Apatch Cordova plugin for WindowsPhone. I need to pass argument from the C# layer to the JavaScript one, and I do it using the PluginResult in the c# layer with formatted string, and with JSON.parse() function in the CordovaCommandResult plugin of the JavaScript layer ( cordova.js file. ) However, when trying to pass string values an exception occurs "Syntax Error: invalid character".
How can I pass string value thus the JSON.parse() will parse it successfully?

Here is my code:

c# layer:

public void GetConfiguration(string sensorName)
{
    PluginResult result = new PluginResult(PluginResult.Status.OK,GetConfiguration());
    DispatchCommandResult(result); 
}

public override string GetConfiguration()
{
    string config = String.Format("\"unit\":{0},\"exponent\":{1},\"frequency\":{2}",
                        unit, 
                        exponent.ToString("0.0", CultureInfo.InvariantCulture), 
                        frequency.ToString("0.0", CultureInfo.InvariantCulture));

    return "{" + config + "}";
}

Where unit is a string variable, and it causes the exception.

JavaScript layer: cordova.js

Calling the GetConfiguration function:

getConfiguration: function (successCallback, errorCallback, args) {            
    var win = function (result) {
        successCallback(result);
    };

    var fail = errorCallback && function (code) {
        errorCallback(code);
    };

    exec(win, fail, "SensorsManager", "GetConfiguration", args);      
},

Parsing the result returned from the c# layer:

  define("cordova/plugin/windows8/CordovaCommandResult", function (require,exports,
  module) {
    var cordova = require('cordova');
    var channel = require('cordova/channel');

    // singular WP callback function attached to window,
    //status is used to determine if it is a success or error
    module.exports = function (status, callbackId, args, cast) {

    if (status === "backbutton") {
        // do not detach backbutton event, as we need to be able to catch exceptions
        cordova.fireDocumentEvent("backbutton", undefined, true);
        return "true";
    }
    else if (status === "resume") {
        cordova.fireDocumentEvent('resume');
        return "true";
    }
    else if (status === "pause") {
        cordova.fireDocumentEvent('pause');
        return "true";
    }

    var parsedArgs;

    try {
        parsedArgs = JSON.parse(args);
    }
    catch (ex) {
        return;
    }

    var safeStatus = parseInt(status, 10);
        if (safeStatus === cordova.callbackStatus.NO_RESULT ||
            safeStatus === cordova.callbackStatus.OK) {
            cordova.callbackSuccess(callbackId, parsedArgs, cast);
        }
        else {
            cordova.callbackError(callbackId, parsedArgs, cast);
        }
    };
});